fix MenuItemButton if child is null (#147485)

Fix https://github.com/flutter/flutter/issues/147479

Pre-launch Checklist

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

@@ -856,7 +856,7 @@ class MenuItemButton extends StatefulWidget {
     this.trailingIcon,
     this.closeOnActivate = true,
     this.overflowAxis = Axis.horizontal,
-    required this.child,
+    this.child,
   });
 
   /// Called when the button is tapped or otherwise activated.
@@ -1141,7 +1141,7 @@ class _MenuItemButtonState extends State<MenuItemButton> {
         trailingIcon: widget.trailingIcon,
         hasSubmenu: false,
         overflowAxis: _anchor?._orientation ?? widget.overflowAxis,
-        child: widget.child!,
+        child: widget.child,
       ),
     );
 
@@ -1986,7 +1986,7 @@ class _SubmenuButtonState extends State<SubmenuButton> {
                 trailingIcon: widget.trailingIcon,
                 hasSubmenu: true,
                 showDecoration: (controller._anchor!._parent?._orientation ?? Axis.horizontal) == Axis.vertical,
-                child: child ?? const SizedBox(),
+                child: child,
               ),
             ),
           ),
@@ -2981,7 +2981,7 @@ class _MenuItemLabel extends StatelessWidget {
     this.shortcut,
     this.semanticsLabel,
     this.overflowAxis = Axis.vertical,
-    required this.child,
+    this.child,
   });
 
   /// Whether or not this menu has a submenu.
@@ -3011,8 +3011,8 @@ class _MenuItemLabel extends StatelessWidget {
   /// The direction in which the menu item expands.
   final Axis overflowAxis;
 
-  /// The required label child widget.
-  final Widget child;
+  /// An optional child widget that is displayed in the label.
+  final Widget? child;
 
   @override
   Widget build(BuildContext context) {
@@ -3029,6 +3029,7 @@ class _MenuItemLabel extends StatelessWidget {
             mainAxisSize: MainAxisSize.min,
             children: <Widget>[
               if (leadingIcon != null) leadingIcon!,
+              if (child != null)
                 Expanded(
                   child: ClipRect(
                     child: Padding(
@@ -3046,6 +3047,7 @@ class _MenuItemLabel extends StatelessWidget {
         mainAxisSize: MainAxisSize.min,
         children: <Widget>[
           if (leadingIcon != null) leadingIcon!,
+          if (child != null)
             Padding(
               padding: leadingIcon != null ? EdgeInsetsDirectional.only(start: horizontalPadding) : EdgeInsets.zero,
               child: child,

packages/flutter/test/material/menu_anchor_test.dart

@@ -2532,6 +2532,22 @@ void main() {
           ..rect(color: overlayColor.withOpacity(0.1)),
       );
     });
+
+    testWidgets('MenuItemButton can build when its child is null', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: SizedBox(
+              width: 200,
+              child: MenuItemButton(),
+            ),
+          ),
+        ),
+      );
+
+      // exception `Null check operator used on a null value` would be thrown.
+      expect(tester.takeException(), isNull);
+    });
   });
 
   group('Layout', () {