You need to initialize the array:
-(instancetype)init {
self = [super init];
if (self) {
_storedLogins = [[NSMutableArray alloc] init];
[self createStoredLogins];
NSLog(@"Stored logins: %@", _storedLogins);
NSLog(@"Stored user: %@", _storedLogins[0][0]);
}
return self;
}