Modifying source prop after initial render throws an exception.

[pfgallagher]

Attempts to modify the source prop in an <Embed/> after the initial render result in the following error being thrown in the browser console:

Exception in event "callback": i is not a function d/this.handleMessage<@https://embed.runkit.com/:86:24

Based on the runkit embed docs, it appears that the underlying api call, notebook.setSource(source, callback), is not receiving the second argument, i.e. callback.