Bug 1727829 - Consider to not initialize AboutReaderChild actor for initial about:blank documents when pagehide fires, r=NeilDeakin

Differential Revision: https://phabricator.services.mozilla.com/D123827

browser/components/BrowserGlue.jsm

@@ -307,7 +307,9 @@ let JSWINDOWACTORS = {
       events: {
         DOMContentLoaded: {},
         pageshow: { mozSystemGroup: true },
-        pagehide: { mozSystemGroup: true },
+        // Don't try to create the actor if only the pagehide event fires.
+        // This can happen with the initial about:blank documents.
+        pagehide: { mozSystemGroup: true, createActor: false },
       },
     },
     messageManagerGroups: ["browsers"],