BugFix: App crashes on iPhone 6S in case of rotation on login screen.

The crash reason was the following:
Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer <RoomViewController> for the key path "isOffline" from <AppDelegate> because it is not registered as an observer.

Vector/ViewController/RoomViewController.m

@@ -236,18 +236,12 @@
     [super viewWillAppear:animated];
     
     [self listenTypingNotifications];
-    
-    // Observe network reachability
-    [[AppDelegate theDelegate]  addObserver:self forKeyPath:@"isOffline" options:0 context:nil];
-    [self refreshActivitiesViewDisplay];
 }
 
 - (void)viewWillDisappear:(BOOL)animated
 {
     [super viewWillDisappear:animated];
     
-    [[AppDelegate theDelegate] removeObserver:self forKeyPath:@"isOffline"];
-    
     // hide action
     if (self.currentAlert)
     {
@@ -286,6 +280,10 @@
     
     [super viewDidAppear:animated];
     
+    // Observe network reachability
+    [[AppDelegate theDelegate]  addObserver:self forKeyPath:@"isOffline" options:0 context:nil];
+    [self refreshActivitiesViewDisplay];
+    
     if (self.roomDataSource)
     {
         // Set visible room id
@@ -297,6 +295,8 @@
 {
     [super viewDidDisappear:animated];
     
+    [[AppDelegate theDelegate] removeObserver:self forKeyPath:@"isOffline"];
+    
     // Reset visible room id
     [AppDelegate theDelegate].visibleRoomId = nil;
 }