Bug 1867886: Ensure that NotificationController can't re-register with the refresh driver after it's been unregistered in Shutdown. r=eeejay

This is just a "belt-and-suspenders" invariant tightening.  Shouldn't impact
behavior at all, assuming that we don't call methods on NotificationController
objects that have been nerfed with Shutdown().

Differential Revision: https://phabricator.services.mozilla.com/D195321

accessible/base/NotificationController.cpp

@@ -92,6 +92,9 @@ void NotificationController::Shutdown() {
   MOZ_RELEASE_ASSERT(mObservingState == eNotObservingRefresh,
                      "Must unregister before being destroyed (and we just "
                      "passed our last change to unregister)");
+  // Immediately null out mPresShell, to prevent us from being registered as a
+  // refresh observer again.
+  mPresShell = nullptr;
 
   // Shutdown handling child documents.
   int32_t childDocCount = mHangingChildDocuments.Length();
@@ -104,7 +107,6 @@ void NotificationController::Shutdown() {
   mHangingChildDocuments.Clear();
 
   mDocument = nullptr;
-  mPresShell = nullptr;
 
   mTextArray.Clear();
   mContentInsertions.Clear();
@@ -459,9 +461,11 @@ void NotificationController::ScheduleContentInsertion(
 }
 
 void NotificationController::ScheduleProcessing() {
-  // If notification flush isn't planed yet start notification flush
+  // If notification flush isn't planned yet, start notification flush
   // asynchronously (after style and layout).
-  if (mObservingState == eNotObservingRefresh) {
+  // Note: the mPresShell null-check might be unnecessary; it's just to prevent
+  // a null-deref here, if we somehow get called after we've been shut down.
+  if (mObservingState == eNotObservingRefresh && mPresShell) {
     if (mPresShell->AddRefreshObserver(this, FlushType::Display,
                                        "Accessibility notifications")) {
       mObservingState = eRefreshObserving;