Bug 1665820, r=emalysz

Differential Revision: https://phabricator.services.mozilla.com/D92589

Author: Kaizer Soze

browser/extensions/screenshots/selector/ui.js

@@ -365,6 +365,7 @@ this.ui = (function() { // eslint-disable-line no-unused-vars
           this.element.setAttribute("role", "dialog");
           this.element.onload = watchFunction(() => {
             msgsPromise.then(([cancelTitle, copyTitle, downloadTitle]) => {
+              assertIsBlankDocument(this.element.contentDocument);
               this.document = this.element.contentDocument;
               this.window = this.element.contentWindow;
               // eslint-disable-next-line no-unsanitized/property