Fix crash when DecrementCapturerCount() makes a delayed WasHidden() call during WebContentsImpl dest

chrome/browser/thumbnails/thumbnail_tab_helper.cc

Index: chrome/browser/thumbnails/thumbnail_tab_helper.cc
diff --git a/chrome/browser/thumbnails/thumbnail_tab_helper.cc b/chrome/browser/thumbnails/thumbnail_tab_helper.cc
index 2ea7feb55c75515656faffbc4e339cd6a1093844..0097afc4ce4634303e01c5a84db603b4e6bd6009 100644
--- a/chrome/browser/thumbnails/thumbnail_tab_helper.cc
+++ b/chrome/browser/thumbnails/thumbnail_tab_helper.cc
@@ -208,7 +208,7 @@ void ThumbnailTabHelper::UpdateThumbnailIfNecessary(
   // Destroying a WebContents may trigger it to be hidden, prompting a snapshot
   // which would be unwise to attempt <http://crbug.com/130097>. If the
   // WebContents is in the middle of destruction, do not risk it.
-  if (web_contents->IsBeingDestroyed())
+  if (!web_contents || web_contents->IsBeingDestroyed())
     return;
   // Skip if a pending entry exists. WidgetHidden can be called while navigating
   // pages and this is not a time when thumbnails should be generated.