--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -1243,18 +1243,19 @@ public:
   {
     MOZ_ASSERT(mTabParent);
   }
 
   NS_IMETHOD Observe(nsISupports* aSubject,
                      const char* aTopic,
                      const char16_t* aData) override
   {
-    if (!mTabParent) {
-      // We already sent the notification
+    if (!mTabParent || !mObserverId) {
+      // We already sent the notification, or we don't actually need to
+      // send any notification at all.
       return NS_OK;
     }
 
     if (!mTabParent->SendNativeSynthesisResponse(mObserverId, nsCString(aTopic))) {
       NS_WARNING("Unable to send native event synthesization response!");
     }
     // Null out tabparent to indicate we already sent the response
     mTabParent = nullptr;