[REG 3.3 -> 3.4] Cannot execute Qt Quick UI project after adding kit

1. Start Creator with enabled QmlProjectManager plugin.
2. Create a new Qt Quick UI project.
3. Go to Projects mode.
4. Add a kit to the project.
5. Make the new kit active.
   The Run button in the lower left corner turns gray.
6. Go to Edit mode.
7. Open a file from the project.
   The Run button in the lower left corner turns green again.

Running the project should all the time be possible by just clicking the Run button.

Found by Squish test tst_qtquick_creation3.