macOS: avoid throwing inside nonBrowserWindowStartup()

On macOS, opening the Page Info window and other "non browser windows" causes a message like this to appear on the Browser Console:

TypeError: document.getElementById(...) is null nonbrowser-mac.js:105:14

This happens because the code inside the nonBrowserWindowStartup() function (in browser/base/content/nonbrowser-mac.js) is blindly looking for a DOM element that is only present in the Mac hidden window. Kathy and I have a patch for this which we should try to upstream to Firefox as well.