Fix null dereference in QuitInstructionBubble

BUG=908677
R=sky

Change-Id: I5d4a5a43fdf6d503cbd69619472e130a7abcc374
Reviewed-on: https://chromium-review.googlesource.com/c/1351690Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611443}

chrome/browser/ui/views/quit_instruction_bubble.cc

@@ -60,9 +60,11 @@ void QuitInstructionBubble::AnimationProgressed(
 
       // Set the bounds to that of the active browser window so that the widget
       // will be centered on the nearest monitor.
-      params.bounds = BrowserView::GetBrowserViewForBrowser(
-                          BrowserList::GetInstance()->GetLastActive())
-                          ->GetBounds();
+      const Browser* last_active = BrowserList::GetInstance()->GetLastActive();
+      if (last_active) {
+        params.bounds =
+            BrowserView::GetBrowserViewForBrowser(last_active)->GetBounds();
+      }
       params.opacity = views::Widget::InitParams::TRANSLUCENT_WINDOW;
       params.ownership = views::Widget::InitParams::WIDGET_OWNS_NATIVE_WIDGET;
       params.accept_events = false;