[Kiosk] Add nullptr check in ArcKioskAppService for the OnAppPrepared() call

There are cases when |delegate_| is set to nullptr, but we are still on the codepath where it is executed.
For example, restart of the application during the session. In such cases, we should not call already
non-existing delegate.

Bug: 1168248
Change-Id: I44bec441d16115f95e9e92452c3beede37fb11a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640406Reviewed-by: Anqing Zhao <anqing@chromium.org>
Commit-Queue: Anatoliy Potapchuk <apotapchuk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845584}

chrome/browser/chromeos/app_mode/arc/arc_kiosk_app_service.cc

@@ -227,6 +227,7 @@ void ArcKioskAppService::PreconditionsChanged() {
       VLOG(2) << "Starting kiosk app";
       app_launcher_ = std::make_unique<ArcKioskAppLauncher>(
           profile_, ArcAppListPrefs::Get(profile_), app_id_, this);
+      if (delegate_)
         delegate_->OnAppPrepared();
     }
   } else if (task_id_ != -1) {