fix(cdk/a11y): error if FocusMonitor is used on non-element nodes (#20640)

* Fixes an error thrown when trying to stop monitoring a non-element node. Now `FocusMonitor.monitor` is a noop.
* Fixes an error on init if the `cdkMonitorFocus` directive is placed on a non-element node.

Related to #20632.

(cherry picked from commit 9edab81)

Author: crisbeto

src/cdk/a11y/focus-monitor/focus-monitor.spec.ts

@@ -351,6 +351,7 @@ describe('cdkMonitorFocus', () => {
         ComplexComponentWithMonitorElementFocus,
         ComplexComponentWithMonitorSubtreeFocus,
         ComplexComponentWithMonitorSubtreeFocusAndMonitorElementFocus,
+        FocusMonitorOnCommentNode,
       ],
     }).compileComponents();
   });
@@ -548,6 +549,14 @@ describe('cdkMonitorFocus', () => {
           expect(childElement.classList).toContain('cdk-keyboard-focused');
         }));
   });
+
+  it('should not throw when trying to monitor focus on a non-element node', () => {
+    expect(() => {
+      const fixture = TestBed.createComponent(FocusMonitorOnCommentNode);
+      fixture.detectChanges();
+      fixture.destroy();
+    }).not.toThrow();
+  });
 });
 
 describe('FocusMonitor observable stream', () => {
@@ -610,7 +619,14 @@ class ComplexComponentWithMonitorElementFocus {}
 })
 class ComplexComponentWithMonitorSubtreeFocus {}
 
+
 @Component({
   template: `<div cdkMonitorSubtreeFocus><button cdkMonitorElementFocus></button></div>`
 })
 class ComplexComponentWithMonitorSubtreeFocusAndMonitorElementFocus {}
+
+
+@Component({
+  template: `<ng-container cdkMonitorElementFocus></ng-container>`
+})
+class FocusMonitorOnCommentNode {}

src/cdk/a11y/focus-monitor/focus-monitor.ts

@@ -225,13 +225,13 @@ export class FocusMonitor implements OnDestroy {
 
   monitor(element: HTMLElement | ElementRef<HTMLElement>,
           checkChildren: boolean = false): Observable<FocusOrigin> {
-    // Do nothing if we're not on the browser platform.
-    if (!this._platform.isBrowser) {
+    const nativeElement = coerceElement(element);
+
+    // Do nothing if we're not on the browser platform or the passed in node isn't an element.
+    if (!this._platform.isBrowser || nativeElement.nodeType !== 1) {
       return observableOf(null);
     }
 
-    const nativeElement = coerceElement(element);
-
     // If the element is inside the shadow DOM, we need to bind our focus/blur listeners to
     // the shadow root, rather than the `document`, because the browser won't emit focus events
     // to the `document`, if focus is moving within the same shadow root.
@@ -570,10 +570,11 @@ export class CdkMonitorFocus implements AfterViewInit, OnDestroy {
   constructor(private _elementRef: ElementRef<HTMLElement>, private _focusMonitor: FocusMonitor) {}
 
   ngAfterViewInit() {
+    const element = this._elementRef.nativeElement;
     this._monitorSubscription = this._focusMonitor.monitor(
-      this._elementRef,
-      this._elementRef.nativeElement.hasAttribute('cdkMonitorSubtreeFocus'))
-      .subscribe(origin => this.cdkFocusChange.emit(origin));
+      element,
+      element.nodeType === 1 && element.hasAttribute('cdkMonitorSubtreeFocus'))
+    .subscribe(origin => this.cdkFocusChange.emit(origin));
   }
 
   ngOnDestroy() {