Ensuring example re-renders always work

[stevematney]

Because the mountNode never changes in the previous version of the code, this.callbackRef would not get called after an error was thrown. After a second re-render with new code, the Example would simply be blank on the screen (not rendering either an Example or an error message).

https://github.com/styleguidist/react-styleguidist/blob/0802ffbe716fef22086f87f5c6a9caec2f9126d1/src/client/rsg-components/Preview/Preview.tsx#L131-L136

This code ensures that we always get an Example re-render when new code is present.