Bug rendering preview component

[lmcorreia]

- Do you want to request a feature or report a bug?
BUG
- What is the current behavior?
An Uncaught Error is thrown while trying to load an entry after working on another (or the same) entry.
- If the current behavior is a bug, please provide the steps to reproduce.
Can be reproduced by checking out master at ab47c3c or newer:

1. In the CMS, click an entry in any collection - the edit and preview panels will load
2. Go back to the collections (Save the entry, Cancel, hit Home icon, ...)
3. Click an entry again (same or other, doesn't matter) - check the Uncaught Error in the console:

Uncaught Error: PreviewPane.render(): A valid React element (or null) must be returned. You may have returned undefined, an array or some other invalid object.
    at invariant (cms.js:5654)
    at ReactCompositeComponentWrapper._renderValidatedComponent (cms.js:59635)
    at ReactCompositeComponentWrapper.performInitialMount (cms.js:59166)
    at ReactCompositeComponentWrapper.mountComponent (cms.js:59062)
    at Object.mountComponent (cms.js:14669)
    at ReactDOMComponent.mountChildren (cms.js:134890)
    at ReactDOMComponent._createInitialChildren (cms.js:132271)
    at ReactDOMComponent.mountComponent (cms.js:132090)
    at Object.mountComponent (cms.js:14669)
    at ReactDOMComponent.mountChildren (cms.js:134890)

- What is the expected behavior?
On step 3 above, for the entry to load as normal.

This seems to have been introduced in ab47c3c but I would appreciate some confirmation. I'm not familiar with React Frame component but if I can be of any help, let me know.