Bug 1669096: Add missing kungFuDeathGrip. r=nika

Differential Revision: https://phabricator.services.mozilla.com/D92490

layout/base/nsDocumentViewer.cpp

@@ -1201,6 +1201,11 @@ bool nsDocumentViewer::GetIsStopped() { return mStopped; }
 NS_IMETHODIMP
 nsDocumentViewer::PermitUnload(PermitUnloadAction aAction,
                                bool* aPermitUnload) {
+  // We're going to be running JS and nested event loops, which could cause our
+  // DocShell to be destroyed. Make sure we stay alive until the end of the
+  // function.
+  RefPtr<nsDocumentViewer> kungFuDeathGrip(this);
+
   if (StaticPrefs::dom_disable_beforeunload()) {
     aAction = eDontPromptAndUnload;
   }