fix(media): check if subscription has been created before unsubscribi…

…ng (#401)

there are cases when ngOnDestroy is executed before ngOnInit can even execute, so we need to validate for that.

src/platform/core/media/directives/media-toggle.directive.ts

@@ -67,7 +67,9 @@ export class TdMediaToggleDirective implements OnInit, OnDestroy {
   }
 
   ngOnDestroy(): void {
-    this._subscription.unsubscribe();
+    if (this._subscription) {
+      this._subscription.unsubscribe();
+    }
   }
 
   private _mediaChange(matches: boolean): void {