Get rid of _NullElement. (flutter#146741)

Fixes flutter#145602

Author: polina-c

packages/flutter/lib/src/cupertino/text_selection_toolbar.dart

@@ -887,13 +887,12 @@ class _CupertinoTextSelectionToolbarItemsElement extends RenderObjectElement {
     _mountChild(toolbarItems.nextButton, _CupertinoTextSelectionToolbarItemsSlot.nextButton);
 
     // Mount list children.
-    _children = List<Element>.filled(toolbarItems.children.length, _NullElement.instance);
     Element? previousChild;
-    for (int i = 0; i < _children.length; i += 1) {
-      final Element newChild = inflateWidget(toolbarItems.children[i], IndexedSlot<Element?>(i, previousChild));
-      _children[i] = newChild;
-      previousChild = newChild;
-    }
+    _children = List<Element>.generate(toolbarItems.children.length, (int i) {
+      final Element result = inflateWidget(toolbarItems.children[i], IndexedSlot<Element?>(i, previousChild));
+      previousChild = result;
+      return result;
+    }, growable: false);
   }
 
   @override
@@ -1273,19 +1272,3 @@ enum _CupertinoTextSelectionToolbarItemsSlot {
   backButton,
   nextButton,
 }
-
-class _NullElement extends Element {
-  _NullElement() : super(const _NullWidget());
-
-  static _NullElement instance = _NullElement();
-
-  @override
-  bool get debugDoingBuild => throw UnimplementedError();
-}
-
-class _NullWidget extends Widget {
-  const _NullWidget();
-
-  @override
-  Element createElement() => throw UnimplementedError();
-}

packages/flutter/test/flutter_test_config.dart

@@ -52,8 +52,6 @@ Future<void> testExecutable(FutureOr<void> Function() testMain) {
       classes: <String>[
         // TODO(polina-c): CurvedAnimation is leaking, https://github.com/flutter/flutter/issues/145600 [leaks-to-clean]
         'CurvedAnimation',
-        // TODO(polina-c): _NullElement is leaking, https://github.com/flutter/flutter/issues/145602 [leaks-to-clean]
-        '_NullElement',
       ],
     );
   }