[Widget Inspector] Handle null exceptions calling renderObject (#163642)

Fixes https://github.com/flutter/devtools/issues/8905

Based on the stacktrace in
https://github.com/flutter/devtools/issues/8905:

* This call to `renderObject` can throw a null-exception:
39b4951f8f/packages/flutter/lib/src/widgets/framework.dart (L3745)
* That exception is thrown here:
39b4951f8f/packages/flutter/lib/src/widgets/framework.dart (L6534)

I've been unable to figure out a way to get into a state that reproduces
this. Instead, this PR simply handles the exception and returns `null`
(because we already gracefully handle the case where the `renderObject`
is `null`.

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

@@ -2068,7 +2068,7 @@ mixin WidgetInspectorService {
       }
       final Object? value = node.value;
       if (value is Element) {
-        final RenderObject? renderObject = value.renderObject;
+        final RenderObject? renderObject = _renderObjectOrNull(value);
         if (renderObject is RenderParagraph) {
           additionalPropertiesJson['textPreview'] = renderObject.text.toPlainText();
         }
@@ -2160,7 +2160,7 @@ mixin WidgetInspectorService {
       return null;
     }
     final RenderObject? renderObject =
-        object is Element ? object.renderObject : (object as RenderObject?);
+        object is Element ? _renderObjectOrNull(object) : (object as RenderObject?);
     if (renderObject == null || !renderObject.attached) {
       return null;
     }
@@ -2224,7 +2224,7 @@ mixin WidgetInspectorService {
           InspectorSerializationDelegate delegate,
         ) {
           final Object? value = node.value;
-          final RenderObject? renderObject = value is Element ? value.renderObject : null;
+          final RenderObject? renderObject = value is Element ? _renderObjectOrNull(value) : null;
           if (renderObject == null) {
             return const <String, Object>{};
           }
@@ -2320,7 +2320,7 @@ mixin WidgetInspectorService {
     final Object? object = toObject(id);
     bool succeed = false;
     if (object != null && object is Element) {
-      final RenderObject? render = object.renderObject;
+      final RenderObject? render = _renderObjectOrNull(object);
       final ParentData? parentData = render?.parentData;
       if (parentData is FlexParentData) {
         parentData.fit = flexFit;
@@ -2338,7 +2338,7 @@ mixin WidgetInspectorService {
     final dynamic object = toObject(id);
     bool succeed = false;
     if (object != null && object is Element) {
-      final RenderObject? render = object.renderObject;
+      final RenderObject? render = _renderObjectOrNull(object);
       final ParentData? parentData = render?.parentData;
       if (parentData is FlexParentData) {
         parentData.flex = factor;
@@ -2362,7 +2362,7 @@ mixin WidgetInspectorService {
     final Object? object = toObject(id);
     bool succeed = false;
     if (object != null && object is Element) {
-      final RenderObject? render = object.renderObject;
+      final RenderObject? render = _renderObjectOrNull(object);
       if (render is RenderFlex) {
         render.mainAxisAlignment = mainAxisAlignment;
         render.crossAxisAlignment = crossAxisAlignment;
@@ -2543,6 +2543,12 @@ mixin WidgetInspectorService {
     _clearStats();
     _resetErrorCount();
   }
+
+  /// Safely get the render object of an [Element].
+  ///
+  /// If the element is not yet mounted, the result will be null.
+  RenderObject? _renderObjectOrNull(Element element) =>
+      element.mounted ? element.renderObject : null;
 }
 
 /// Accumulator for a count associated with a specific source location.

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -4837,6 +4837,32 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
         expect(parentData['offsetY'], equals('293.0'));
       });
 
+      testWidgets(
+        'ext.flutter.inspector.getLayoutExplorerNode does not throw for unmounted widget',
+        (WidgetTester tester) async {
+          // Mount the Row widget.
+          await pumpWidgetForLayoutExplorer(tester);
+
+          // Get the id of the Row widget.
+          final Element rowElement = tester.element(find.byType(Row));
+          service.setSelection(rowElement, group);
+          final String id = service.toId(rowElement, group)!;
+
+          // Unmount the Row widget.
+          await tester.pumpWidget(const Placeholder());
+
+          // Verify that the call to getLayoutExplorerNode for the Row widget
+          // does not throw an exception.
+          expect(
+            () => service.testExtension(
+              WidgetInspectorServiceExtensions.getLayoutExplorerNode.name,
+              <String, String>{'id': id, 'groupName': group, 'subtreeDepth': '1'},
+            ),
+            returnsNormally,
+          );
+        },
+      );
+
       testWidgets('ext.flutter.inspector.getLayoutExplorerNode for RenderBox with FlexParentData', (
         WidgetTester tester,
       ) async {