Fix crash for showing app list in invalid display

Changes:
Fix the crash by doing early return when no root window exists for the
specified display.

Bug: 903293
Test: AppListPresenterDelegateTest.ShowInInvalidDisplay
Change-Id: I866f07e037c269851ab6c72fff622a0c7c27a4ac
Reviewed-on: https://chromium-review.googlesource.com/c/1341000
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609367}

ash/app_list/app_list_presenter_delegate_unittest.cc

@@ -1088,6 +1088,13 @@ TEST_F(AppListPresenterDelegateTest, SearchBoxShownOnSmallDisplay) {
   EXPECT_LE(0, view->GetWidget()->GetNativeView()->bounds().y());
 }
 
+// Tests that no crash occurs after an attempt to show app list in an invalid
+// display.
+TEST_F(AppListPresenterDelegateTest, ShowInInvalidDisplay) {
+  GetAppListTestHelper()->ShowAndRunLoop(display::kInvalidDisplayId);
+  GetAppListTestHelper()->CheckState(app_list::AppListViewState::CLOSED);
+}
+
 // Test a variety of behaviors for home launcher (app list in tablet mode).
 class AppListPresenterDelegateHomeLauncherTest
     : public AppListPresenterDelegateTest {

ash/app_list/presenter/app_list_presenter_impl.cc

@@ -126,6 +126,11 @@ void AppListPresenterImpl::Show(int64_t display_id,
     return;
   }
 
+  if (!delegate_->GetRootWindowForDisplayId(display_id)) {
+    LOG(ERROR) << "Root window does not exist for display: " << display_id;
+    return;
+  }
+
   is_visible_ = true;
   RequestPresentationTime(display_id, event_time_stamp);
 

ash/app_list/presenter/app_list_presenter_impl_unittest.cc

@@ -70,7 +70,7 @@ class AppListPresenterDelegateTest : public AppListPresenterDelegate {
   bool IsHomeLauncherEnabledInTabletMode() override { return false; }
   bool GetOnScreenKeyboardShown() override { return false; }
   aura::Window* GetRootWindowForDisplayId(int64_t display_id) override {
-    return nullptr;
+    return container_->GetRootWindow();
   }
   void OnVisibilityChanged(bool visible, aura::Window* root_window) override {}
   void OnTargetVisibilityChanged(bool visible) override {}