Fix null child for FittedBox (#86662)

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -2651,11 +2651,11 @@ class RenderFittedBox extends RenderProxyBox {
 
   @override
   void paint(PaintingContext context, Offset offset) {
-    if (size.isEmpty || child!.size.isEmpty)
+    if (child == null || size.isEmpty || child!.size.isEmpty)
       return;
     _updatePaintData();
-    if (child != null) {
-      if (_hasVisualOverflow! && clipBehavior != Clip.none)
+    assert(child != null);
+    if (_hasVisualOverflow! && clipBehavior != Clip.none) {
       layer = context.pushClipRect(
         needsCompositing,
         offset,
@@ -2664,7 +2664,7 @@ class RenderFittedBox extends RenderProxyBox {
         oldLayer: layer is ClipRectLayer ? layer! as ClipRectLayer : null,
         clipBehavior: clipBehavior,
       );
-      else
+    } else {
       layer = _paintChildWithTransform(context, offset);
     }
   }

packages/flutter/test/widgets/fitted_box_test.dart

@@ -589,6 +589,24 @@ void main() {
 
     expect(outsideBox.size.height, 50.0);
   });
+
+  testWidgets('FittedBox without child does not throw', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const Center(
+        child: SizedBox(
+          width: 200.0,
+          height: 200.0,
+          child: FittedBox(),
+        ),
+      ),
+    );
+
+    expect(find.byType(FittedBox), findsOneWidget);
+
+    // Tapping it also should not throw.
+    await tester.tap(find.byType(FittedBox), warnIfMissed: false);
+    expect(tester.takeException(), isNull);
+  });
 }
 
 List<Type> getLayers() {