Can open Settings dialog twice, leading to crash

To reproduce:

Open a project in Creator, set up a build configuration with a Qt which has no debugging helpers.
Start debugging this project with cdb.
While the project is being built, open the settings dialog.

When building the project finished, a dialog will pop up warning about missing debugging helpers.
Click "Open Qt preferences". The settings dialog will open for a second time. Close it and debugging will start.
Close the other open settings dialog and Creator will crash