Speculative fix for folder crash

Folder crashes during closing animation. |activated_folder_item_view_|
may be nullptr at that time, so fix it by checking nullptr.

BUG=813211

Change-Id: Iff04425a5178d55222195dc44742a83dfe50ae63
Reviewed-on: https://chromium-review.googlesource.com/990714Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547529}

ui/app_list/views/app_list_folder_view.cc

@@ -159,12 +159,16 @@ class FolderItemTitleAnimation : public gfx::SlideAnimation,
  private:
   // gfx::AnimationDelegate
   void AnimationProgressed(const gfx::Animation* animation) override {
+    if (!folder_view_->GetActivatedFolderItemView())
+      return;
     folder_view_->GetActivatedFolderItemView()->title()->SetEnabledColor(
         gfx::Tween::ColorValueBetween(animation->GetCurrentValue(), from_color_,
                                       to_color_));
   }
 
   void AnimationEnded(const gfx::Animation* animation) override {
+    if (!folder_view_->GetActivatedFolderItemView())
+      return;
     folder_view_->GetActivatedFolderItemView()->title()->SetEnabledColor(
         to_color_);
     folder_view_->RecordAnimationSmoothness();