[Sharesheet] Missing null check.

Bug: 1126788
Change-Id: I8c632beddf21fa93740c6a4e76b6d385c940530f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439998Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812510}

chrome/browser/ui/views/sharesheet/sharesheet_bubble_view.cc

@@ -285,8 +285,10 @@ void SharesheetBubbleView::ButtonPressed(views::Button* sender,
                                 std::move(intent_), share_action_view_);
     intent_.reset();
     user_cancelled_ = false;
+    if (close_callback_) {
       std::move(close_callback_).Run(sharesheet::SharesheetResult::kSuccess);
     }
+  }
 }
 
 std::unique_ptr<views::NonClientFrameView>