Update parse_live_list.dart (#406)

In theory this should not make any difference. In practise sometimes an error gets thrown:
The following NoSuchMethodError was thrown while finalizing the widget tree:
The method 'dispose' was called on null.
Receiver: null
Tried calling: dispose()

Author: fischerscode

lib/src/utils/parse_live_list.dart

@@ -894,7 +894,7 @@ class _ParseLiveListWidgetState<T extends ParseObject>
 
   @override
   void dispose() {
-    _liveList.dispose();
+    _liveList?.dispose();
     _liveList = null;
     super.dispose();
   }