Check MultiUserWindowManager before setting window's owner.

When adding a new user, although IsMultiProfileAvailable return true,
MultiUserWindowManagerHelper::GetWindowManager is still null. So add a
check for MultiUserWindowManagerHelper::GetWindowManager. When
MultiUserWindowManagerHelper::GetWindowManager is still null, we don't
need set owner for windows, because multiple user window manager is not
ready yet.

BUG=1047376

Change-Id: Idb527b24e0f7d66714b6f408b08f20e50b6f8c32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032486Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737225}

chrome/browser/apps/app_service/extension_apps.cc

@@ -783,9 +783,13 @@ void ExtensionApps::OnAppWindowAdded(extensions::AppWindow* app_window) {
   // Attach window to multi-user manager now to let it manage visibility state
   // of the window correctly.
   if (SessionControllerClientImpl::IsMultiProfileAvailable()) {
-    MultiUserWindowManagerHelper::GetWindowManager()->SetWindowOwner(
-        app_window->GetNativeWindow(),
-        multi_user_util::GetAccountIdFromProfile(profile_));
+    auto* multi_user_window_manager =
+        MultiUserWindowManagerHelper::GetWindowManager();
+    if (multi_user_window_manager) {
+      multi_user_window_manager->SetWindowOwner(
+          app_window->GetNativeWindow(),
+          multi_user_util::GetAccountIdFromProfile(profile_));
+    }
   }
   RegisterInstance(app_window, InstanceState::kStarted);
 }