[REG 4.5 -> 4.6] "Go to slot" does not work

1. Create a new Qt Widgets Application project.
2. Open the project's .ui file.
3. In Design mode, drag a Push Button into the window.
4. Right-click on the button and select "Go to slot..." from the context menu.
5. Select "clicked()" and click "OK".
6. The slot will not be shown. You'll see an error message:
   

This worked correctly in Creator 4.5.

Found and tested by Squish test tst_designer_goto_slot.