fix a MaterialApp NNBD issue (#81529)

packages/flutter/lib/src/material/app.dart

@@ -815,7 +815,7 @@ class _MaterialAppState extends State<MaterialApp> {
                 return widget.builder!(context, child);
               },
             )
-          : child!,
+          : child ?? const SizedBox.shrink(),
       ),
     );
   }

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

@@ -1170,7 +1170,7 @@ class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
       assert(!_usesRouter);
       _defaultRouteInformationProvider?.dispose();
       _defaultRouteInformationProvider = null;
-      if (oldWidget == null || widget.navigatorKey != oldWidget.navigatorKey) {
+      if (_navigator == null || widget.navigatorKey != oldWidget!.navigatorKey) {
         _navigator = widget.navigatorKey ?? GlobalObjectKey<NavigatorState>(this);
       }
       assert(_navigator != null);

packages/flutter/test/material/material_test.dart

@@ -64,6 +64,20 @@ class ElevationColor {
 }
 
 void main() {
+  // Regression test for https://github.com/flutter/flutter/issues/81504
+  testWidgets('MaterialApp.home nullable and update test', (WidgetTester tester) async {
+    // _WidgetsAppState._usesNavigator == true
+    await tester.pumpWidget(const MaterialApp(home: SizedBox.shrink()));
+
+    // _WidgetsAppState._usesNavigator == false
+    await tester.pumpWidget(const MaterialApp()); // Do not crash!
+
+    // _WidgetsAppState._usesNavigator == true
+    await tester.pumpWidget(const MaterialApp(home: SizedBox.shrink())); // Do not crash!
+
+    expect(tester.takeException(), null);
+  });
+
   testWidgets('default Material debugFillProperties', (WidgetTester tester) async {
     final DiagnosticPropertiesBuilder builder = DiagnosticPropertiesBuilder();
     const Material().debugFillProperties(builder);