Creator shows source instead of translated string

1. Build Creator from current 2.7 branch.
   I used 530b2f79eb7c4203df9c122b44744646099c8879
2. Run this build in German language.
3. Open a file.
4. Hover the mouse-cursor over the "X" in the upper right corner of the editor.
   The tooltip "Close Document" will be shown.

Instead, Creator should show the German translation.

What's strange is that the source string is enclosed in "tr()" and has been translated in the German ts/qm files.