Fix an issue where the sidebar failed to appear in Safari 11 and Chrome
63 when returning a cross-origin Window from a Promise `then` callback.
In these browsers an exception is triggered when the browser tries to
test if the return value is a Promise that can be unwrapped.

The issue was resolved in more recent browsers by adding an undefined
`then` property to cross origin objects. [1]

[1] https://github.com/whatwg/dom/issues/536