Modify the updateChildren method deep copy _children (#120773)

* Modify the updateChildren method deep copy _children

* add test

* fix some small nits

* Simplified newChildren declaration in updateChildren

packages/flutter/lib/src/widgets/framework.dart

@@ -5944,8 +5944,7 @@ abstract class RenderObjectElement extends Element {
     int newChildrenBottom = newWidgets.length - 1;
     int oldChildrenBottom = oldChildren.length - 1;
 
-    final List<Element> newChildren = oldChildren.length == newWidgets.length ?
-        oldChildren : List<Element>.filled(newWidgets.length, _NullElement.instance);
+    final List<Element> newChildren = List<Element>.filled(newWidgets.length, _NullElement.instance);
 
     Element? previousChild;
 

packages/flutter/test/widgets/framework_test.dart

@@ -1753,6 +1753,30 @@ The findRenderObject() method was called for the following element:
     child.dependOnInheritedElement(ancestor);
     expect(child.doesDependOnInheritedElement(ancestor), isTrue);
   });
+
+  testWidgets(
+      'MultiChildRenderObjectElement.updateChildren test',
+      (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/120762.
+    final GlobalKey globalKey = GlobalKey();
+    await tester.pumpWidget(Column(
+      children: <Widget>[
+        const SizedBox(),
+        SizedBox(key: globalKey),
+        const SizedBox(),
+      ],
+    ));
+    expect(tester.takeException(), isNull);
+
+    await tester.pumpWidget(Column(
+      children: <Widget>[
+        const SizedBox(),
+        const SizedBox(),
+        SizedBox(child: SizedBox(key: globalKey)),
+      ],
+    ));
+    expect(tester.takeException(), isNull);
+  });
 }
 
 class _TestInheritedElement extends InheritedElement {