Forbid calling findRenderObject on an unmounted element (#83962)

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

@@ -2056,7 +2056,8 @@ abstract class BuildContext {
   /// This method will only return a valid result after the build phase is
   /// complete. It is therefore not valid to call this from a build method.
   /// It should only be called from interaction event handlers (e.g.
-  /// gesture callbacks) or layout or paint callbacks.
+  /// gesture callbacks) or layout or paint callbacks. It is also not valid to
+  /// call if [State.mounted] returns false.
   ///
   /// If the render object is a [RenderBox], which is the common case, then the
   /// size of the render object can be obtained from the [size] getter. This is
@@ -3864,7 +3865,25 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
   }
 
   @override
-  RenderObject? findRenderObject() => renderObject;
+  RenderObject? findRenderObject() {
+    assert(() {
+      if (_lifecycleState != _ElementLifecycle.active) {
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('Cannot get renderObject of inactive element.'),
+          ErrorDescription(
+            'In order for an element to have a valid renderObject, it must be '
+            'active, which means it is part of the tree.\n'
+            'Instead, this element is in the $_lifecycleState state.\n'
+            'If you called this method from a State object, consider guarding '
+            'it with State.mounted.',
+          ),
+          describeElement('The findRenderObject() method was called for the following element'),
+        ]);
+      }
+      return true;
+    }());
+    return renderObject;
+  }
 
   @override
   Size? get size {

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

@@ -2455,6 +2455,11 @@ class InspectorSelection {
 
   Element? _currentElement;
   set currentElement(Element? element) {
+    if (element?.debugIsDefunct == true) {
+      _currentElement = null;
+      _current = null;
+      return;
+    }
     if (currentElement != element) {
       _currentElement = element;
       _current = element!.findRenderObject();

packages/flutter/test/widgets/framework_test.dart

@@ -1619,6 +1619,30 @@ void main() {
     await pumpWidget(Container());
     expect(states, <String>['deactivate', 'dispose']);
   });
+
+  testWidgets('Getting the render object of an unmounted element throws', (WidgetTester tester) async {
+    await tester.pumpWidget(const _StatefulLeaf());
+    final StatefulElement element = tester.element<StatefulElement>(find.byType(_StatefulLeaf));
+    expect(element.state, isA<State<_StatefulLeaf>>());
+    expect(element.widget, isA<_StatefulLeaf>());
+    // Replace the widget tree to unmount the element.
+    await tester.pumpWidget(Container());
+
+  expect(
+    () => element.findRenderObject(),
+    throwsA(isA<FlutterError>().having(
+      (FlutterError error) => error.message,
+      'message',
+      equalsIgnoringHashCodes('''
+Cannot get renderObject of inactive element.
+In order for an element to have a valid renderObject, it must be active, which means it is part of the tree.
+Instead, this element is in the _ElementLifecycle.defunct state.
+If you called this method from a State object, consider guarding it with State.mounted.
+The findRenderObject() method was called for the following element:
+  StatefulElement#00000(DEFUNCT)'''),
+      )),
+    );
+  });
 }
 
 class _WidgetWithNoVisitChildren extends StatelessWidget {