exception on ext-notifications.js

prevents-notifications-exception-on-unloaded-extensions.patch

diff --git a/toolkit/components/extensions/ext-notifications.js b/toolkit/components/extensions/ext-notifications.js
--- a/toolkit/components/extensions/ext-notifications.js
+++ b/toolkit/components/extensions/ext-notifications.js
@@ -47,24 +47,31 @@ Notification.prototype = {
       svc.closeAlert(this.id);
     } catch (e) {
       // This will fail if the OS doesn't support this function.
     }
     notificationsMap.get(this.extension).delete(this.id);
   },
 
   observe(subject, topic, data) {
+    let notifications = notificationsMap.get(this.extension);
+    if (!notifications) {
+      // no notifications found for the addon, e.g. the addon has been
+      // already unloaded.
+      return;
+    }
+
     if (topic === "alertclickcallback") {
-      notificationsMap.get(this.extension).emit("clicked", data);
+      notifications.emit("clicked", data);
     }
     if (topic === "alertfinished") {
-      notificationsMap.get(this.extension).emit("closed", data);
+      notifications.emit("closed", data);
     }
 
-    notificationsMap.get(this.extension).delete(this.id);
+    notifications.delete(this.id);
   },
 };
 
 /* eslint-disable mozilla/balanced-listeners */
 extensions.on("startup", (type, extension) => {
   let map = new Map();
   EventEmitter.decorate(map);
   notificationsMap.set(extension, map);