qt: use correct key when removing the invalidated sg graphics api cache

As the comment there says, this is not really important because askToQuit() is going to make the application wait for the initial (asynchronous) probing (which probes and caches the appropriate sg graphics api for the next startup) anyway. But still, this a mistake and should be corrected.

I also start to classify scene graph error critical and not as warning.

Request review @chub.