Bug 1853498 - Make "Save image as" from context menu of RSS item work when displayed as web page. r=freaktechnik

Differential Revision: https://phabricator.services.mozilla.com/D190159

--HG--
extra : rebase_source : 592669e9cc9a1c67a327e28ea9378a1cce073072

mail/base/content/mailContext.js

@@ -544,7 +544,7 @@ var mailContextMenu = {
           false, // skipPrompt
           null, // referrerInfo
           null, // cookieJarSettings
-          this.browsingContext.window.document, // sourceDocument
+          this.browsingContext.window?.document, // sourceDocument
           null, // isContentWindowPrivate,
           Services.scriptSecurityManager.getSystemPrincipal() // principal
         );