use FlutterError in MultiChildRenderObjectWidget (#37187)

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

@@ -1688,8 +1688,9 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
       assert(() {
         final int index = children.indexOf(null);
         if (index >= 0) {
-          throw AssertionError(
-            "The widget's children must not contain any null values, but a null value was found at index $index"
+          throw FlutterError(
+            "$runtimeType's children must not contain any null values, "
+            'but a null value was found at index $index'
           );
         }
         return true;

packages/flutter/test/widgets/multichild_test.dart

@@ -31,6 +31,13 @@ void checkTree(WidgetTester tester, List<BoxDecoration> expectedDecorations) {
   }
 }
 
+class MockMultiChildRenderObjectWidget extends MultiChildRenderObjectWidget {
+  MockMultiChildRenderObjectWidget({ Key key, List<Widget> children }) : super(key: key, children: children);
+
+  @override
+  RenderObject createRenderObject(BuildContext context) => null;
+}
+
 void main() {
   testWidgets('MultiChildRenderObjectElement control test', (WidgetTester tester) async {
 
@@ -345,4 +352,17 @@ void main() {
 
     checkTree(tester, <BoxDecoration>[kBoxDecorationB, kBoxDecorationC]);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/37136.
+  test('provides useful assertion message when one of the children is null', () {
+    bool assertionTriggered = false;
+    try {
+      MockMultiChildRenderObjectWidget(children: const <Widget>[null]);
+    } catch (e) {
+      expect(e.toString(), contains("MockMultiChildRenderObjectWidget's children must not contain any null values,"));
+      assertionTriggered = true;
+    }
+
+    expect(assertionTriggered, isTrue);
+  });
 }