When removing a project setStartupProject(nullptr) may fail, since
it will reject any attempt to set the startupProject to nullptr while
any project is still available.

So delay this till nullptr is an option:-)

Change-Id: I6f41a17d922cf6333b7b7e17f8300e831e4807d9
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>