Fix(tree):allow NestedTreeControl to have null or undefined child

src/cdk/tree/control/base-tree-control.ts

@@ -34,7 +34,7 @@ export abstract class BaseTreeControl<T> implements TreeControl<T> {
   isExpandable: (dataNode: T) => boolean;
 
   /** Gets a stream that emits whenever the given data node's children change. */
-  getChildren: (dataNode: T) => (Observable<T[]> | T[]);
+  getChildren: (dataNode: T) => (Observable<T[]> | T[] | undefined | null);
 
   /** Toggles one single data node's expanded/collapsed state. */
   toggle(dataNode: T): void {

src/cdk/tree/control/nested-tree-control.ts

@@ -13,7 +13,7 @@ import {BaseTreeControl} from './base-tree-control';
 export class NestedTreeControl<T> extends BaseTreeControl<T> {
 
   /** Construct with nested tree function getChildren. */
-  constructor(public getChildren: (dataNode: T) => (Observable<T[]> | T[])) {
+  constructor(public getChildren: (dataNode: T) => (Observable<T[]> | T[] | undefined | null)) {
     super();
   }
 

src/cdk/tree/control/tree-control.ts

@@ -60,5 +60,5 @@ export interface TreeControl<T> {
   readonly isExpandable: (dataNode: T) => boolean;
 
   /** Gets a stream that emits whenever the given data node's children change. */
-  readonly getChildren: (dataNode: T) => Observable<T[]> | T[];
+  readonly getChildren: (dataNode: T) => Observable<T[]> | T[] | undefined | null;
 }

src/lib/tree/tree.spec.ts

@@ -237,6 +237,26 @@ describe('MatTree', () => {
     });
   });
 
+  describe('nested tree with undefined or null children', () => {
+    describe('should initialize', () => {
+      let fixture: ComponentFixture<MatNestedTreeWithNullOrUndefinedChild >;
+
+      beforeEach(() => {
+        configureMatTreeTestingModule([MatNestedTreeWithNullOrUndefinedChild]);
+        fixture = TestBed.createComponent(MatNestedTreeWithNullOrUndefinedChild);
+        treeElement = fixture.nativeElement.querySelector('mat-tree');
+
+        fixture.detectChanges();
+      });
+
+      it('with rendered dataNodes', () => {
+        const nodes = getNodes(treeElement);
+
+        expect(nodes).toBeDefined('Expect nodes to be defined');
+        expect(nodes[0].classList).toContain('customNodeClass');
+      });
+    });
+  });
   describe('nested tree', () => {
     describe('should initialize', () => {
       let fixture: ComponentFixture<NestedMatTreeApp>;
@@ -710,6 +730,29 @@ class MatTreeWithNullOrUndefinedChild {
   hasChild = (_: number, node: ExampleFlatNode) => node.expandable;
 }
 
+@Component({
+  template: `
+    <mat-tree [dataSource]="dataSource" [treeControl]="treeControl">
+      <mat-nested-tree-node *matTreeNodeDef="let node" class="customNodeClass">
+        {{node.name}}
+        <ng-template matTreeNodeOutlet></ng-template>
+      </mat-nested-tree-node>
+    </mat-tree>
+  `
+})
+class MatNestedTreeWithNullOrUndefinedChild {
+  treeControl: NestedTreeControl<FoodNode>;
+  dataSource: MatTreeNestedDataSource<FoodNode>;
+
+  constructor() {
+    this.treeControl = new NestedTreeControl<FoodNode>(this.getChildren);
+    this.dataSource = new MatTreeNestedDataSource();
+    this.dataSource.data = TREE_DATA;
+  }
+
+  private getChildren = (node: FoodNode) => node.children;
+}
+
 @Component({
   template: `
     <mat-tree [dataSource]="dataSource" [treeControl]="treeControl">

tools/public_api_guard/cdk/tree.d.ts

@@ -1,7 +1,7 @@
 export declare abstract class BaseTreeControl<T> implements TreeControl<T> {
     dataNodes: T[];
     expansionModel: SelectionModel<T>;
-    getChildren: (dataNode: T) => (Observable<T[]> | T[]);
+    getChildren: (dataNode: T) => (Observable<T[]> | T[] | undefined | null);
     getLevel: (dataNode: T) => number;
     isExpandable: (dataNode: T) => boolean;
     collapse(dataNode: T): void;
@@ -127,8 +127,8 @@ export declare function getTreeMultipleDefaultNodeDefsError(): Error;
 export declare function getTreeNoValidDataSourceError(): Error;
 
 export declare class NestedTreeControl<T> extends BaseTreeControl<T> {
-    getChildren: (dataNode: T) => (Observable<T[]> | T[]);
-    constructor(getChildren: (dataNode: T) => (Observable<T[]> | T[]));
+    getChildren: (dataNode: T) => (Observable<T[]> | T[] | undefined | null);
+    constructor(getChildren: (dataNode: T) => (Observable<T[]> | T[] | undefined | null));
     protected _getDescendants(descendants: T[], dataNode: T): void;
     expandAll(): void;
     getDescendants(dataNode: T): T[];
@@ -137,7 +137,7 @@ export declare class NestedTreeControl<T> extends BaseTreeControl<T> {
 export interface TreeControl<T> {
     dataNodes: T[];
     expansionModel: SelectionModel<T>;
-    readonly getChildren: (dataNode: T) => Observable<T[]> | T[];
+    readonly getChildren: (dataNode: T) => Observable<T[]> | T[] | undefined | null;
     readonly getLevel: (dataNode: T) => number;
     readonly isExpandable: (dataNode: T) => boolean;
     collapse(dataNode: T): void;