App can crash on Login if a secondary view is presented before beginning authentication

[pixlwave]

The AuthenticationViewController will try to deregister it's observer on the activity indicator twice:

2021-07-26 14:19:22.197476+0100 Riot[47607:1159140] *** Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer <AuthenticationViewController 0x7fdbd4836c00> for the key path "hidden" from <UIActivityIndicatorView 0x7fdbd3a27210> because it is not registered as an observer.'
*** First throw call stack:
(
	0   CoreFoundation                      0x000000011318afba __exceptionPreprocess + 242
	1   libobjc.A.dylib                     0x0000000112762ff5 objc_exception_throw + 48
	2   Foundation                          0x000000010b05fee4 -[NSObject(NSKeyValueObserverRegistration) _removeObserver:forProperty:] + 689
	3   Foundation                          0x000000010b0602de -[NSObject(NSKeyValueObserverRegistration) removeObserver:forKeyPath:] + 74
	4   Riot                                0x0000000107bc8cf6 -[AuthenticationViewController viewDidDisappear:] + 134
	5   UIKitCore                           0x000000011a62df03 -[UIViewController _setViewAppearState:isAnimating:] + 370
	6   UIKitCore                           0x000000011a62ecb3 -[UIViewController __viewDidDisappear:] + 131