chore: more null checks

Author: crisbeto

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

@@ -27,7 +27,10 @@ export class FocusTrap {
   get enabled(): boolean { return this._enabled; }
   set enabled(val: boolean) {
     this._enabled = val;
-    this._startAnchor.tabIndex = this._endAnchor.tabIndex = this._enabled ? 0 : -1;
+
+    if (this._startAnchor && this._endAnchor) {
+      this._startAnchor.tabIndex = this._endAnchor.tabIndex = this._enabled ? 0 : -1;
+    }
   }
   private _enabled: boolean = true;
 
@@ -44,11 +47,11 @@ export class FocusTrap {
 
   /** Destroys the focus trap by cleaning up the anchors. */
   destroy() {
-    if (this._startAnchor.parentNode) {
+    if (this._startAnchor && this._startAnchor.parentNode) {
       this._startAnchor.parentNode.removeChild(this._startAnchor);
     }
 
-    if (this._endAnchor.parentNode) {
+    if (this._endAnchor && this._endAnchor.parentNode) {
       this._endAnchor.parentNode.removeChild(this._endAnchor);
     }