catch schema mod in background

If the schema confirmation dialog is shown on a background thread it
will crash the app, so we convert this to an assertion error that gets
caught by the error handler. Code still needs to be updated to modify
the schema prior to moving to the background, but at least this way
it doesn't crash.

qt/aqt/main.py

@@ -1283,6 +1283,7 @@ and if the problem comes up again, please ask on the support site."""
     ##########################################################################
 
     def onSchemaMod(self, arg):
+        assert self.inMainThread()
         progress_shown = self.progress.busy()
         if progress_shown:
             self.progress.finish()