Bug in the .ui file

[wysota]

So I guess that is some kind of bug in QtCreator... no reason, in terms of code, for this to happen.

It's not a bug in Qt Creator. Somewhere in your file you used a character that is invalid in context of XML. While copy&pasting, your system somehow corrected it to the proper character or discarded the (possibly not-printable) character.

Well, last question: is there a difference between

<widget class="QComboBox" name="GraphToolbarCharttype"/>

and

<widget class="QComboBox" name="GraphToolbarCharttype">

Yes. The second one opens a tag called widget while the first one both opens and closes a tag called widget.