Fix null dereference in ExtensionsGuestViewManagerDelegate.

extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc

Index: extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc
diff --git a/extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc b/extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc
index 49461b052b5bd3469f3896def332d3c3846c5e59..061480b5acd6431aa9abb7e02c98ebe5e881d7f9 100644
--- a/extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc
+++ b/extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc
@@ -57,6 +57,9 @@ void ExtensionsGuestViewManagerDelegate::DispatchEvent(
                                               << " must have a histogram value";
 
   content::WebContents* owner = guest->owner_web_contents();
+  if (!owner)
+    return;  // Could happen at tab shutdown.
+
   EventRouter::DispatchEventToSender(owner, guest->browser_context(),
                                      guest->owner_host(), histogram_value,
                                      event_name, std::move(event_args),