OK, I had this, and it irked me too. I don't know what happened to cause this, but it is not serious. I suppose that breaking and remaking all the layouts might fix it. Other frameworks tend toward a lot of warnings, but tho I'm new to Qt, not a one yet. I went in another editor, and removed a line that said (as best I can remember):
<z-order>verticalSpacer</z-order>
which was among a lot of other lines which also were z-order tags. I deleted the line with this tag, and rebuilt all. Problem gone. Interestingly enough, all of the z-order tags had vanished from my file when I looked at it next. It must be a bug, but evidently one of little consequence; except those who hate to see the serene beauty of Qt spoiled.
"
, but''
(two empty single quotes, that signify an empty string), and the tag to kill is an empty<zorder></zorder>
tag (as explained in the answers that followed). – Zielsdorf