I installed Android Studio 4.0. Added Flutter plugin. Installed Flutter already. Flutter doctor also showed everything is running well.
> C:\Users\Alvin>flutter doctor
> Doctor summary (to see all details, run flutter doctor -v):
> [√] Flutter (Channel beta, 1.19.0-4.1.pre, on Microsoft Windows [Version 10.0.18363.900], locale en-US)
> [√] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
> [√] Android Studio (version 4.0) [√] VS Code (version 1.45.1)
> [√] Connected device (1 available)
>
> • No issues found!
But when I open AVD Manager and launch the AVD in the emulator. I also got a pop-up 
But actually the adb is installed already in C:\Users\Alvin\AppData\Local\Android\Sdk\platform-tools. So I add the path into the environment variable PATH to include this directory. And I can call the adb from anywhere in the command prompt.
But the same "Unable to locate adb still pop up every time I launch AVD. Actually, if I generate a simple Flutter program and run it in a mobile connected to the USB, it work in the mobile device.
Based on curiosity, I try to figure out what is actually happening. I opened the log file idea.log (open using Help -> Show log in explorer). I found out that the system actually trying to find ANDROID_HOME and ANDROID_SDK_ROOT. After I set them up, the Android Studio seems to be running well but there is a message that is still annoying. .deployment.AsyncDevicesGetter - adb not found Even though it is INFO only, I suspect this is the same problem when launching AVD Manager. the list of the log is as follows
2020-06-19 22:50:39,009 [ 0] INFO - #com.intellij.idea.Main - ------------------------------------------------------ IDE STARTED ------------------------------------------------------
2020-06-19 22:50:39,213 [ 204] INFO - #com.intellij.idea.Main - JNA library (64-bit) loaded in 154 ms
2020-06-19 22:50:39,610 [ 601] INFO - #com.intellij.idea.Main - IDE: Android Studio (build #AI-193.6911.18.40.6514223, 20 May 2020 22:07)
2020-06-19 22:50:39,610 [ 601] INFO - #com.intellij.idea.Main - OS: Windows 10 (10.0, amd64)
2020-06-19 22:50:39,610 [ 601] INFO - #com.intellij.idea.Main - JRE: 1.8.0_242-release-1644-b01 (JetBrains s.r.o)
2020-06-19 22:50:39,610 [ 601] INFO - #com.intellij.idea.Main - JVM: 25.242-b01 (OpenJDK 64-Bit Server VM)
2020-06-19 22:50:39,614 [ 605] INFO - #com.intellij.idea.Main - JVM Args: exit -Xms256m -Xmx1280m -XX:ReservedCodeCacheSize=240m -XX:+UseConcMarkSweepGC -XX:SoftRefLRUPolicyMSPerMB=50 -XX:CICompilerCount=2 -Dsun.io.useCanonPrefixCache=false -Djava.net.preferIPv4Stack=true -Djdk.http.auth.tunneling.disabledSchemes="" -Djdk.attach.allowAttachSelf=true -Dkotlinx.coroutines.debug=off -Djdk.module.illegalAccess.silent=true -Djna.nosys=true -Djna.boot.library.path= -Djb.vmOptionsFile=C:\Program Files\Android\Android Studio\bin\studio64.exe.vmoptions -Djava.library.path=C:\Program Files\Android\Android Studio\jre\jre\bin;C:\Program Files\Android\Android Studio\jre\jre\bin\server -Didea.platform.prefix=AndroidStudio -Didea.jre.check=true -Dide.native.launcher=true -Didea.paths.selector=AndroidStudio4.0 -XX:ErrorFile=C:\Users\Alvin\java_error_in_studio_%p.log -XX:HeapDumpPath=C:\Users\Alvin\java_error_in_studio.hprof
2020-06-19 22:50:39,618 [ 609] INFO - #com.intellij.idea.Main - ext: C:\Program Files\Android\Android Studio\jre\jre\lib\ext: [access-bridge-64.jar, cldrdata.jar, dnsns.jar, jaccess.jar, localedata.jar, meta-index, nashorn.jar, sunec.jar, sunjce_provider.jar, sunmscapi.jar, sunpkcs11.jar, zipfs.jar]
2020-06-19 22:50:39,618 [ 609] INFO - #com.intellij.idea.Main - charsets: JNU=Cp1252 file=Cp1252
2020-06-19 22:50:39,635 [ 626] INFO - ntellij.idea.ApplicationLoader - CPU cores: 4; ForkJoinPool.commonPool: java.util.concurrent.ForkJoinPool@c001ec7[Running, parallelism = 3, size = 0, active = 0, running = 0, steals = 0, tasks = 0, submissions = 0]; factory: com.intellij.concurrency.IdeaForkJoinWorkerThreadFactory@d88ff34
2020-06-19 22:50:39,985 [ 976] INFO - penapi.util.io.win32.IdeaWin32 - Native filesystem for Windows is operational
2020-06-19 22:50:40,976 [ 1967] INFO - llij.ide.plugins.PluginManager - Plugin "Groovy" misses optional descriptor duplicates-groovy.xml
2020-06-19 22:50:40,978 [ 1969] INFO - llij.ide.plugins.PluginManager - Plugin "Groovy" misses optional descriptor duplicates-detection-groovy.xml
2020-06-19 22:50:41,360 [ 2351] INFO - llij.ide.plugins.PluginManager - Plugin "Java" misses optional descriptor profiler-java.xml
2020-06-19 22:50:41,532 [ 2523] INFO - llij.ide.plugins.PluginManager - Loaded bundled plugins: Android APK Support (1.0), Android Games (1.0), Android NDK Support (1.0), Android Support (10.4.0), App Links Assistant (0.1), ChangeReminder (193.6911.18.40.6514223), Compose (0.1.0-dev03), Configuration Script (193.6911.18.40.6514223), Copyright (193.6911.18.40.6514223), Coverage (193.6911.18.40.6514223), EditorConfig (193.6911.18.40.6514223), Firebase App Indexing (0.1), Firebase Services (0.1), Firebase Testing (1.0), Git (193.6911.18.40.6514223), GitHub (193.6911.18.40.6514223), Google Cloud Tools Core (0.2.8), Google Cloud Tools For Android Studio (0.2.8), Google Developers Samples (0.3.1), Google Login (1.0), Google Services (0.1), Gradle (193.6911.18.40.6514223), Gradle-Java (193.6911.18.40.6514223), Groovy (193.6911.18.40.6514223), IDEA CORE (193.6911.18.40.6514223), IntelliLang (193.6911.18.40.6514223), JUnit (193.6911.18.40.6514223), Java (193.6911.18.40.6514223), Java Bytecode Decompiler (193.6911.18.40.6514223), Java IDE Customization (193.6911.18.40.6514223), Java Internationalization (193.6911.18.40.6514223), Java Stream Debugger (193.6911.18.40.6514223), Kotlin (1.3.72-release-Studio4.0-1), Layoutlib Native (1.0), Layoutlib Standard (1.0), Mercurial (193.6911.18.40.6514223), Properties (193.6911.18.40.6514223), Settings Repository (193.6911.18.40.6514223), Shell Script (193.6911.18.40.6514223), Smali Support (193.6911.18.40.6514223), Subversion (193.6911.18.40.6514223), Task Management (193.6911.18.40.6514223), Terminal (193.6911.18.40.6514223), Test Recorder (1.0), TestNG (193.6911.18.40.6514223), TextMate bundles (193.6911.18.40.6514223), YAML (193.6911.18.40.6514223)
2020-06-19 22:50:41,532 [ 2523] INFO - llij.ide.plugins.PluginManager - Loaded custom plugins: Dart (193.7361), Flutter (46.0.2)
2020-06-19 22:50:43,023 [ 4014] INFO - ellij.util.io.PagedFileStorage - lower=100; upper=500; buffer=10; max=1226
2020-06-19 22:50:43,053 [ 4044] INFO - tellij.util.io.FileChannelUtil - interruptible FileChannel-s will be used for indexes
2020-06-19 22:50:43,295 [ 4286] INFO - rains.ide.BuiltInServerManager - built-in server started, port 63342
2020-06-19 22:50:43,425 [ 4416] INFO - til.net.ssl.CertificateManager - Default SSL context initialized
2020-06-19 22:50:43,444 [ 4435] INFO - pl.local.NativeFileWatcherImpl - Starting file watcher: C:\Program Files\Android\Android Studio\bin\fsnotifier64.exe
2020-06-19 22:50:43,529 [ 4520] INFO - pl.local.NativeFileWatcherImpl - Native file watcher is operational.
2020-06-19 22:50:43,558 [ 4549] INFO - com.intellij.ide.ui.UISettings - Loaded: fontSize=13, fontScale=1.0; restored: fontSize=13, fontScale=1.0
2020-06-19 22:50:43,597 [ 4588] INFO - gs.impl.UpdateCheckerComponent - channel: release
2020-06-19 22:50:44,105 [ 5096] WARN - nSystem.impl.ActionManagerImpl - keymap "ReSharper" not found [Plugin: com.android.tools.ndk]
2020-06-19 22:50:45,429 [ 6420] WARN - ins.android.sdk.AndroidSdkData - Observed package id 'platform-tools' in inconsistent location 'C:\Users\Alvin\AppData\Local\Android\Sdk\platform-tools.old' (Expected 'C:\Users\Alvin\AppData\Local\Android\Sdk\platform-tools')
2020-06-19 22:50:45,429 [ 6420] WARN - ins.android.sdk.AndroidSdkData - Already observed package id 'platform-tools' in 'C:\Users\Alvin\AppData\Local\Android\Sdk\platform-tools'. Skipping duplicate at 'C:\Users\Alvin\AppData\Local\Android\Sdk\platform-tools.old'
2020-06-19 22:50:45,650 [ 6641] INFO - pl$FileIndexDataInitialization - Initialization done: 1992
2020-06-19 22:50:46,465 [ 7456] INFO - exImpl$StubIndexInitialization - Initialization done: 799
2020-06-19 22:50:50,013 [ 11004] INFO - artup.AndroidStudioInitializer - Failed to disable 'org.intellij.plugins.intelliLang.inject.groovy.GrConcatenationInjector'
2020-06-19 22:50:50,175 [ 11166] INFO - j.ide.script.IdeStartupScripts - 0 startup script(s) found
2020-06-19 22:50:51,671 [ 12662] WARN - ugins.textmate.TextMateService - Missing builtin bundles, checked:
C:/Users/Alvin/.AndroidStudio4.0/config/plugins/textmate/lib/bundles
C:/Program Files/Android/Android Studio/plugins/textmate/lib/bundles
2020-06-19 22:50:51,712 [ 12703] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:50:52,412 [ 13403] INFO - lutter.run.daemon.DeviceDaemon - starting Flutter device daemon #1: C:/flutterSDK/flutter/bin/flutter.bat daemon
2020-06-19 22:50:52,420 [ 13411] INFO - #io.flutter.sdk.FlutterCommand - C:\flutterSDK\flutter\bin\flutter.bat [--no-color, config, --machine]
2020-06-19 22:50:52,441 [ 13432] INFO - #io.flutter.sdk.FlutterSdk - Calling config --machine
2020-06-19 22:50:53,707 [ 14698] INFO - .diagnostic.PerformanceWatcher - Pushing properties took 493ms; general responsiveness: ok; EDT responsiveness: ok
2020-06-19 22:50:53,804 [ 14795] WARN - com.intellij.util.xmlb.Binding - no accessors for class org.jetbrains.kotlin.idea.highlighter.KotlinDefaultHighlightingSettingsProvider
2020-06-19 22:50:54,741 [ 15732] INFO - .diagnostic.PerformanceWatcher - Indexable file iteration took 1032ms; general responsiveness: ok; EDT responsiveness: ok
2020-06-19 22:50:55,874 [ 16865] INFO - #com.jetbrains.cidr.lang - Clearing symbols finished in 588 ms
2020-06-19 22:50:56,113 [ 17104] INFO - #com.jetbrains.cidr.lang - Building symbols in FAST mode, 0 source files from total 0 project files
2020-06-19 22:50:56,597 [ 17588] INFO - ngd.lsp.ClangDaemonContextImpl - Using clangd from: C:\Program Files\Android\Android Studio\bin\clang\win\clangd.exe
2020-06-19 22:50:56,651 [ 17642] INFO - #com.jetbrains.cidr.lang - Loading symbols finished in 647 ms
2020-06-19 22:50:56,678 [ 17669] INFO - #com.jetbrains.cidr.lang - Loading Header Maps finished in 27 ms
2020-06-19 22:50:56,730 [ 17721] INFO - #io.flutter.sdk.FlutterCommand - C:\flutterSDK\flutter\bin\flutter.bat [--no-color, config, --machine]
2020-06-19 22:50:56,733 [ 17724] INFO - #io.flutter.sdk.FlutterSdk - Calling config --machine
2020-06-19 22:50:57,254 [ 18245] INFO - #com.jetbrains.cidr.lang - Loading Module Maps finished in 543 ms
2020-06-19 22:50:57,279 [ 18270] INFO - .diagnostic.PerformanceWatcher - Post-startup activities under progress took 5678ms; general responsiveness: 1/6 sluggish; EDT responsiveness: 3/5 sluggish
2020-06-19 22:50:57,282 [ 18273] INFO - #com.jetbrains.cidr.lang - Loaded 0 module maps in 0 packs for 0 search roots
2020-06-19 22:50:57,304 [ 18295] INFO - #com.jetbrains.cidr.lang - Saved 0 module maps in 0 packs
2020-06-19 22:50:57,326 [ 18317] INFO - #com.jetbrains.cidr.lang - Saving Module Maps finished in 23 ms
2020-06-19 22:50:57,326 [ 18317] INFO - #com.jetbrains.cidr.lang - Loaded 0 tables for 0 files (0 project files)
2020-06-19 22:50:57,327 [ 18318] INFO - #com.jetbrains.cidr.lang - Building symbols for 0 source files
2020-06-19 22:50:57,360 [ 18351] INFO - #com.jetbrains.cidr.lang - Updating Symbols finished in 24 ms
2020-06-19 22:50:57,363 [ 18354] INFO - #com.jetbrains.cidr.lang - Building symbols for 0 unused headers
2020-06-19 22:50:57,368 [ 18359] INFO - #com.jetbrains.cidr.lang - Updating Symbols finished in 5 ms
2020-06-19 22:50:57,368 [ 18359] INFO - #com.jetbrains.cidr.lang - Updating Symbols finished in 5 ms
2020-06-19 22:50:57,402 [ 18393] INFO - #com.jetbrains.cidr.lang - Saving modified symbols for 0 files (0 tables of total 0)
2020-06-19 22:50:57,414 [ 18405] INFO - #com.jetbrains.cidr.lang - Saving symbols finished in 12 ms
2020-06-19 22:50:57,446 [ 18437] INFO - #io.flutter.sdk.FlutterSdk - Timeout when calling flutter config --machine
2020-06-19 22:50:57,481 [ 18472] INFO - tartup.impl.StartupManagerImpl - C:/Projects/android/Flutter/first_flutter_app/.idea case-sensitivity: expected=false actual=false
2020-06-19 22:50:57,694 [ 18685] INFO - ge.ExternalProjectsDataStorage - Load external projects data in 8 millis (read time: 1)
2020-06-19 22:50:57,827 [ 18818] INFO - tor.impl.FileEditorManagerImpl - Project opening took 11239 ms
2020-06-19 22:50:57,872 [ 18863] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:50:58,214 [ 19205] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:50:58,909 [ 19900] INFO - CompilerWorkspaceConfiguration - Available processors: 4
2020-06-19 22:50:58,946 [ 19937] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:50:59,228 [ 20219] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:50:59,730 [ 20721] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:00,306 [ 21297] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:00,759 [ 21750] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:01,274 [ 22265] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:01,783 [ 22774] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:02,222 [ 23213] INFO - #io.flutter.sdk.FlutterSdk - Timeout when calling flutter config --machine
2020-06-19 22:51:02,288 [ 23279] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:02,789 [ 23780] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:03,640 [ 24631] INFO - j.ide.plugins.RepositoryHelper - using cached plugin list (updated at 6/19/2020 8:30 PM)
2020-06-19 22:51:06,694 [ 27685] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:07,458 [ 28449] INFO - j.ide.actions.RevealFileAction -
Exit code 1
2020-06-19 22:51:09,822 [ 30813] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:15,262 [ 36253] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:15,362 [ 36353] INFO - mponents.impl.stores.StoreUtil - saveProjectsAndApp took 1126 ms
2020-06-19 22:51:19,857 [ 40848] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:51:59,968 [ 80959] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:52:04,295 [ 85286] INFO - rationStore.ComponentStoreImpl - Saving appHttpConfigurable took 16 ms
2020-06-19 22:52:04,326 [ 85317] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:52:04,342 [ 85333] INFO - rationStore.ComponentStoreImpl - Saving Project (name=first_flutter_app, containerState=ACTIVE, componentStore=C:\Projects\android\Flutter\first_flutter_app) ProjectCodeStyleConfiguration took 16 ms
2020-06-19 22:56:13,286 [ 334277] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:13,786 [ 334777] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:14,301 [ 335292] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:14,817 [ 335808] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:15,332 [ 336323] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:17,379 [ 338370] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:19,925 [ 340916] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:20,956 [ 341947] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:21,472 [ 342463] INFO - .deployment.AsyncDevicesGetter - adb not found
2020-06-19 22:56:23,504 [ 344495] INFO - .deployment.AsyncDevicesGetter - adb not found
Are there any information that can be shared about what I am facing and how to fix it? Additional information: adb version : 30.0.3-6597393 java version: 1.8.0_121