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).

react-styleguidist/src/client/rsg-components/Preview/Preview.tsx

Lines 131 to 136 in 0802ffb

	return (
	<>
	<div data-testid="mountNode" ref={this.callbackRef} />
	{error && <PlaygroundError message={error} />}
	</>
	);

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

[stevematney]

Resolved by #2158