fix remaining search term when emptying field too fast

lib/pages/user_list/user_list_page.dart

@@ -61,7 +61,11 @@ class _UserListPageState extends State<UserListPage> {
                       title: TextField(
                         controller: _searchController,
                         onChanged: (String searchTerm) {
-                          setState(() => _lastSearchTerm = searchTerm);
+                          if (searchTerm.isEmpty) {
+                            _onCancelSearch();
+                          } else {
+                            setState(() => _lastSearchTerm = searchTerm);
+                          }
                         },
                         autocorrect: false,
                         decoration: InputDecoration(