msglist: dispose of resources created for ScrollToBottomButton in MessageList

Author: sirpengi

lib/widgets/message_list.dart

@@ -135,6 +135,8 @@ class _MessageListState extends State<MessageList> {
   @override
   void dispose() {
     model?.dispose();
+    scrollController.dispose();
+    _scrollToBottomVisibleValue.dispose();
     super.dispose();
   }