Bugfix cleaning up the interference of flats creation with the modal dialog for primary scope check

The additional check that the correct scope is used for imaging introduced with commit 9869bcfb had the unpleasant side effect that it interfered with flats creation. This fix corrects the event handling of the modal dialog that appears when the CCD does not use the primary scope.

Test scenario: Create a LRGB sequence of flats. Without this fix, the modal dialog "Cover telescope with evenly illuminated light source." appears twice and capturing fails. After applying this fix, flats should work again as expected.