Fix issue on slow connections

lib/ui/screens/conversation_screen.dart

@@ -63,7 +63,9 @@ class _ConversationScreenState extends State<ConversationScreen> {
   @override
   void deactivate() {
     super.deactivate();
-    _refreshTime.cancel();
+
+    if(_refreshTime != null)
+      _refreshTime.cancel();
   }
 
   void _setError(ErrorLevel err) => setState(() => _error = err);