Bug 1599304: Make SpecialPowers.Focus check for availability of this.manager.rootFrameLoader before dereferencing it. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D54648

--HG--
extra : moz-landing-system : lando

testing/specialpowers/content/SpecialPowersParent.jsm

@@ -577,7 +577,9 @@ class SpecialPowersParent extends JSWindowActorParent {
         return undefined;
 
       case "SpecialPowers.Focus":
+        if (this.manager.rootFrameLoader) {
           this.manager.rootFrameLoader.ownerElement.focus();
+        }
         return undefined;
 
       case "SpecialPowers.CreateFiles":