Avoid runtime error when placed inside TabBarView

lib/src/widgets/toolbar/arrow_indicated_button_list.dart

@@ -60,6 +60,8 @@ class _ArrowIndicatedButtonListState extends State<ArrowIndicatedButtonList>
   }
 
   void _handleScroll() {
+    if (!mounted) return;
+
     setState(() {
       _showLeftArrow =
           _controller.position.minScrollExtent != _controller.position.pixels;