Add checking for app_connection_holder to avoid core dump in test case.

BUG=992839

Change-Id: Ifc4f0674b8509552dca61d07fe1bbb30c654a877
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1750467Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686327}

chrome/browser/apps/app_service/arc_apps.cc

@@ -527,12 +527,14 @@ void ArcApps::LoadIconFromVM(const std::string app_id,
 
       AppConnectionHolder* app_connection_holder =
           prefs->app_connection_holder();
-      if (app_connection_holder->IsConnected()) {
-        std::move(pending).Run(app_connection_holder);
-      } else {
-        pending_load_icon_calls_.push_back(std::move(pending));
+      if (app_connection_holder) {
+        if (app_connection_holder->IsConnected()) {
+          std::move(pending).Run(app_connection_holder);
+        } else {
+          pending_load_icon_calls_.push_back(std::move(pending));
+        }
+        return;
       }
-      return;
     }
   }