The preview-as-visitor function throws a JSON error on public instances.

To reproduce:

1. Log into a public instance AdminCentral.
2. Open the Preview as visitor app. --> JSON error

elemental.json.impl.JreJsonNull cannot be cast to elemental.json.JsonString.
RpcInvocationException: Unable to invoke method call in com.vaadin.ui.JavaScript$JavaScriptCallbackRpc
caused by InvocationTargetException
caused by ClassCastException: elemental.json.impl.JreJsonNull cannot be cast to elemental.json.JsonString

Full stack trace attached: stacktrace.txt.

The error is specific to public instances. It is not thrown on an author instance.

The error was discovered by a client who wanted to verify that their variants were published/exported correctly. So there is a valid use case why previewing as visitor should work on public instances. It is not an author-instance-only task.

The error is not related to travel demo content. I was able to reproduce it without demo modules too.