uncaught exception: Target applet or JVM process exited abruptly - from a firefox extension

Question

I have written a Firefox extension. When a menuitem is selected, it checks for Java version. I am getting this exception, when I try to call a Java class inside JavaScript:

uncaught exception: Target applet or JVM process exited abruptly

This is happening after I hibernate my system, if I try to use my Firefox extension then I get this error. If I restart Firefox, it works again. What can I do about it?

Sample code for Firefox extension (overlay.xul):

<?xml version="1.0"?>
<overlay id="RefMArc-overlay"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <script>
    function onRclick_temp() {

        try {
            //chkJava: java.lang.String;
            var vers = java.lang.System.getProperty("java.version");
            var ind1 = vers.indexOf(".")+1;
            vers = vers.substring(ind1, vers.indexOf(".", ind1));


            if( vers < 5 ) {
                alert("You need to upgrade java");
                return;
            }

            // to do

        } catch(e) {
            alert("error:" + e);
        }

    }
    <menupopup id="menu_ToolsPopup">

      <menuitem id="helloworld-hello" label="helloworld" 
                oncommand="onMenuClick();" class="menuitem-iconic"/>
    </menupopup>
  </toolbox>
</overlay>

Emma