Fix _LateInitializationError for RenderObjectElement.renderObject (#70974)

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

@@ -5463,8 +5463,8 @@ abstract class RenderObjectElement extends Element {
 
   /// The underlying [RenderObject] for this element.
   @override
-  RenderObject get renderObject => _renderObject;
-  late RenderObject _renderObject;
+  RenderObject get renderObject => _renderObject!;
+  RenderObject? _renderObject;
 
   bool _debugDoingBuild = false;
   @override
@@ -5566,7 +5566,7 @@ abstract class RenderObjectElement extends Element {
 
   void _debugUpdateRenderObjectOwner() {
     assert(() {
-      _renderObject.debugCreator = DebugCreator(this);
+      renderObject.debugCreator = DebugCreator(this);
       return true;
     }());
   }
@@ -6058,7 +6058,7 @@ abstract class RenderObjectElement extends Element {
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
-    properties.add(DiagnosticsProperty<RenderObject>('renderObject', renderObject, defaultValue: null));
+    properties.add(DiagnosticsProperty<RenderObject>('renderObject', _renderObject, defaultValue: null));
   }
 }
 

packages/flutter/test/widgets/framework_test.dart

@@ -1494,6 +1494,12 @@ void main() {
     expect(GestureBinding.instance!.pointerRouter.debugGlobalRouteCount, pointerRouterCount);
     expect(RawKeyboard.instance.keyEventHandler, same(rawKeyEventHandler));
   });
+
+  testWidgets('Can access debugFillProperties without _LateInitializationError', (WidgetTester tester) async {
+    final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
+    TestRenderObjectElement().debugFillProperties(builder);
+    expect(builder.properties.any((DiagnosticsNode property) => property.name == 'renderObject' && property.value == null), isTrue);
+  });
 }
 
 class _FakeFocusManager implements FocusManager {
@@ -1837,3 +1843,7 @@ class FakeLeafRenderObject extends RenderBox {
     size = constraints.biggest;
   }
 }
+
+class TestRenderObjectElement extends RenderObjectElement {
+  TestRenderObjectElement() : super(Table());
+}