Handle channel without preview config better

Due to errors the preview config of a channel can be missing. The channel manager does not handle this situation very well.

Example: a custom event handler aborted creating the preview configuration (see 1). The user gets a message that edit mode could not be entered. After that, clicking on the "Channels" breadcrumb at the bottom does not work anymore. You must refresh the browser first to be able to work again (on 13.1,  on 12.6 the channels overview is shown).

In case there is an internal link from a document in a channel to a document in the un-editable channel, you can click on that link and the channel manager WILL now show you the non-editable channel. This should not be possible.

How to deal with this situation?

(1) https://documentation.bloomreach.com/library/concepts/channels/channel-manager-events.html