Visual Studio crashes when opening ui files

If at least one of the Qt Versions defined in the "Qt Options" dialog is missing, trying to open a form file (*.ui) by double clicking it in the solution explorer causes Visual Studio to crash.

Steps to reproduce:

1. Open Extensions / Qt VS Tools / Qt Options
2. Add a Qt Version
3. Open the Windows Explorer and remove / rename the folder of the Qt version we just added
4. Restart Visual Studio
5. Open a project that contains a form file (or create one)
6. Double click on the form file in the Solution Explorer

Visual Studio will crash after a short delay. This also happens if there are still valid Qt Versions. It suffices if one of them is broken.