[NG] Add focus method guard to FocusTrap

- Addresses use cases for IE11 where the previousActiveELement may not have the focus method
- Closes #1587

Signed-off-by: Matt Hippely <mhippely@vmware.com>

src/clarity-angular/utils/focus-trap/focus-trap.directive.ts

@@ -33,7 +33,9 @@ export class FocusTrapDirective implements AfterViewInit, OnDestroy {
     }
 
     public setPreviousFocus(): void {
-        this._previousActiveElement.focus();
+        if (this._previousActiveElement && this._previousActiveElement.focus) {
+            this._previousActiveElement.focus();
+        }
     }
 
     ngOnDestroy() {