fixed warning

lib/pages/task_edit_page.dart

@@ -168,7 +168,7 @@ class _TaskEditPageState extends State<TaskEditPage> {
           ],
         ),
       ).then((result) {
-        if (result != null && result && context.mounted) {
+        if (result != null && result && mounted) {
           Navigator.of(context).pop();
         }
       });