In dispose unregister from TextEditingController

In dispose unregister from TextEditingController listeners,
otherwise the code might crash if the external TextEditingController
has a longer lifetime than the SearchableList.

lib/searchable_listview.dart

@@ -472,14 +472,13 @@ class _SearchableListState<T> extends State<SearchableList<T>> {
         });
       }
     });
-    widget.searchTextController?.addListener(() {
-      filterList(widget.searchTextController?.text ?? '');
-    });
+    widget.searchTextController?.addListener(_textControllerListener);
   }
 
   @override
   void dispose() {
     scrollController.dispose();
+    widget.searchTextController?.removeListener(_textControllerListener);
     super.dispose();
   }
 
@@ -995,4 +994,8 @@ class _SearchableListState<T> extends State<SearchableList<T>> {
       });
     }
   }
+
+  void _textControllerListener() {
+    filterList(widget.searchTextController?.text ?? '');
+  }
 }