Returning null in a GeneratePreviewURL function creates type and runtime errors

[evelynhathaway]

Link to reproduction

https://payloadcms.com/docs/configuration/collections#preview:~:text=Example%20collection%20with%20preview%20function

To Reproduce

Use the example preview collection function on any collection.

Describe the Bug

TypeScript errors on the return of null.

Type '(document: Partial<CollectionType>, { locale }: GeneratePreviewURLOptions) => string | null' is not assignable to type 'GeneratePreviewURL'.
  Type 'string | null' is not assignable to type 'string | Promise<string>'.
    Type 'null' is not assignable to type 'string | Promise<string>'.

The preview button still appears in the Admin UI, and clicking it opens a not found page. It tries to load the URL null relative to the current page, which redirects to the generic not found page for the collection.

I'd expect that the example function would not have a TypeScript issue and returning null would hide the button or at least let the user know that the preview is not available for the current document and its fields.

Payload Version

1.6.28

[JessChowdhury]

Hi @evelynhathaway - I haven't been able to reproduce this on 1.6.32, if you update to this version (payload@latest) do you still see the type error?

Definitely agree with you on the preview button, it would be great to either hide or clearly show it as disabled when you return null - this will likely be a feature request rather than an issue, I'll see what the team thinks here.

[jacobsfletch]

@evelynhathaway you were right in that our docs incorrectly indicated that null could be returned from admin.preview—I just opened a PR to support this. Once released, a toast notification is thrown when a null preview URL is generated after clicking that button.

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