I can change the other default apps, but when I try to change the photo viewer app, the settings window closes and the option I clicked isn't set. It still says "TWINUI".

The reason I want to change the photo viewer is because I can't open any photos with TWINUI. It says "Class not registered". I always have to go to Open With > Photo Gallery in order to look at pictures. I've looked at my photos so many times before. Why can't I do it now? And why can't I change the default app?