fix: anchors being created too soon

Author: crisbeto

src/lib/core/a11y/focus-trap.ts

@@ -20,8 +20,8 @@ import {coerceBooleanProperty} from '../coercion/boolean-property';
  * This will be replaced with a more intelligent solution before the library is considered stable.
  */
 export class FocusTrap {
-  private _startAnchor: HTMLElement = this._createAnchor();
-  private _endAnchor: HTMLElement = this._createAnchor();
+  private _startAnchor: HTMLElement;
+  private _endAnchor: HTMLElement;
 
   /** Whether the focus trap is active. */
   get enabled(): boolean { return this._enabled; }
@@ -60,6 +60,14 @@ export class FocusTrap {
    * in the constructor, but can be deferred for cases like directives with `*ngIf`.
    */
   attachAnchors(): void {
+    if (!this._startAnchor) {
+      this._startAnchor = this._createAnchor();
+    }
+
+    if (!this._endAnchor) {
+      this._endAnchor = this._createAnchor();
+    }
+
     this._ngZone.runOutsideAngular(() => {
       this._element
         .insertAdjacentElement('beforebegin', this._startAnchor)