fix(core): NestedStack defaultChild is undefined (#20450)

fixes #11221
----

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packages/@aws-cdk/core/lib/nested-stack.ts

@@ -140,6 +140,7 @@ export class NestedStack extends Stack {
     this.resource.applyRemovalPolicy(props.removalPolicy ?? RemovalPolicy.DESTROY);
 
     this.nestedStackResource = this.resource;
+    this.node.defaultChild = this.resource;
 
     // context-aware stack name: if resolved from within this stack, return AWS::StackName
     // if resolved from the outer stack, use the { Ref } of the AWS::CloudFormation::Stack resource

packages/@aws-cdk/core/test/nested-stack.test.ts

@@ -0,0 +1,25 @@
+import {
+  Stack, NestedStack, CfnStack,
+} from '../lib';
+import { toCloudFormation } from './util';
+
+describe('nested-stack', () => {
+  test('a nested-stack has a defaultChild', () => {
+    const stack = new Stack();
+    var nestedStack = new NestedStack(stack, 'MyNestedStack');
+    var cfn_nestedStack = (nestedStack.node.defaultChild) as CfnStack;
+    cfn_nestedStack.addPropertyOverride('TemplateURL', 'http://my-url.com');
+    expect(toCloudFormation(stack)).toEqual({
+      Resources: {
+        MyNestedStackNestedStackMyNestedStackNestedStackResource9C617903: {
+          DeletionPolicy: 'Delete',
+          Properties: {
+            TemplateURL: 'http://my-url.com',
+          },
+          Type: 'AWS::CloudFormation::Stack',
+          UpdateReplacePolicy: 'Delete',
+        },
+      },
+    });
+  });
+});