Login Screen: Add nullptr checks for that there is a next focusable view.

If there are no focusable views in the LockContentsView a crash will occur when
toggling focus. Although this doesn't currently cause a crash, it was a problem
in the past and we need to prevent it from happening in case of UI changes in
the future.

TEST=unit test
BUG=914103

Change-Id: Iee5f23d98aa202998ccd91c743e1f0279c162c81
Reviewed-on: https://chromium-review.googlesource.com/c/1493446Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636541}

ash/login/ui/lock_contents_view.cc

@@ -134,15 +134,15 @@ class AuthErrorLearnMoreButton : public views::Button,
   DISALLOW_COPY_AND_ASSIGN(AuthErrorLearnMoreButton);
 };
 
-// Returns the first or last focusable child of |root|. If |reverse| is false,
-// this returns the first focusable child. If |reverse| is true, this returns
+// Focuses the first or last focusable child of |root|. If |reverse| is false,
+// this focuses the first focusable child. If |reverse| is true, this focuses
 // the last focusable child.
-views::View* FindFirstOrLastFocusableChild(views::View* root, bool reverse) {
+void FocusFirstOrLastFocusableChild(views::View* root, bool reverse) {
   views::FocusSearch search(root, reverse /*cycle*/,
                             false /*accessibility_mode*/);
   views::FocusTraversable* dummy_focus_traversable;
   views::View* dummy_focus_traversable_view;
-  return search.FindNextFocusableView(
+  views::View* focusable_view = search.FindNextFocusableView(
       root,
       reverse ? views::FocusSearch::SearchDirection::kBackwards
               : views::FocusSearch::SearchDirection::kForwards,
@@ -150,6 +150,8 @@ views::View* FindFirstOrLastFocusableChild(views::View* root, bool reverse) {
       views::FocusSearch::StartingViewPolicy::kSkipStartingView,
       views::FocusSearch::AnchoredDialogPolicy::kCanGoIntoAnchoredDialog,
       &dummy_focus_traversable, &dummy_focus_traversable_view);
+  if (focusable_view)
+    focusable_view->RequestFocus();
 }
 
 // Make a section of the text bold.
@@ -1092,7 +1094,7 @@ void LockContentsView::OnFocusLeavingLockScreenApps(bool reverse) {
   if (!reverse || lock_screen_apps_active_)
     FocusNextWidget(reverse);
   else
-    FindFirstOrLastFocusableChild(this, reverse)->RequestFocus();
+    FocusFirstOrLastFocusableChild(this, reverse);
 }
 
 void LockContentsView::OnOobeDialogStateChanged(mojom::OobeDialogState state) {
@@ -1108,7 +1110,7 @@ void LockContentsView::OnFocusLeavingSystemTray(bool reverse) {
   // from the system shelf (or tray) - lock shelf view expect the focus to be
   // taken when it passes it to lock screen view, and can misbehave in case the
   // focus is kept in it.
-  FindFirstOrLastFocusableChild(this, reverse)->RequestFocus();
+  FocusFirstOrLastFocusableChild(this, reverse);
 
   if (lock_screen_apps_active_) {
     Shell::Get()->login_screen_controller()->FocusLockScreenApps(reverse);

ash/login/ui/lock_contents_view_unittest.cc

@@ -2184,4 +2184,18 @@ TEST_F(LockContentsViewUnitTest, RightAndLeftAcceleratorsWithNoUser) {
   lock->AcceleratorPressed(ui::Accelerator(ui::VKEY_LEFT, 0));
 }
 
+TEST_F(LockContentsViewUnitTest, OnFocusLeavingSystemTrayWithNoUsers) {
+  auto* lock = new LockContentsView(
+      mojom::TrayActionState::kNotAvailable, LockScreen::ScreenType::kLock,
+      DataDispatcher(),
+      std::make_unique<FakeLoginDetachableBaseModel>(DataDispatcher()));
+  SetWidget(CreateWidgetWithContent(lock));
+
+  // Check that there is always a focusable view after transitioning focus.
+  lock->OnFocusLeavingSystemTray(false /* reverse */);
+  EXPECT_TRUE(lock->GetFocusManager()->GetFocusedView());
+  lock->OnFocusLeavingSystemTray(true /* reverse */);
+  EXPECT_TRUE(lock->GetFocusManager()->GetFocusedView());
+}
+
 }  // namespace ash