Umbraco-cms: v7.15.0 Preview does not work

[mariojsnunes]

Upgraded from 7.13.2 to 7.15.0 and the preview stopped working.
Downgraded to 7.14 and it is now working again.

Reproduction

Just click on the preview button on an instance of v7.15.0

I did some debugging and found that this method returns null:

Which caused this exception to be thrown:

I obviously did run the health check which was successful, but the problem persisted.
Note that I tried upgrading and downgrading multiple times. Preview never works on v7.15.0, but works on previous versions.

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_This item has been added to our backlog AB#1794_

[stevemegson]

Ah, so that's where that exception comes from.

At least for the site I'm seeing it on, it's being caused by deleted items being returned by the SQL in ContentRepository.BuildPreviewXmlCache. That confuses the code that finds the parent for each item, and it calls AppendChild on a null parent.

[bergmania]

Hi @mariojsnunes..

I have a hard time reproducing this.

7.15 has a new preview engine. I assume you run with the default value of Umbraco.Preview.Mode in app settings (If not specified)? - Can you please check that it works if you use the old preview engine, by setting the value to Legacy.

Do you have the problem on all content items or only on some of them - If so, can you show how the document type looks?

[svanleusden]

We were having the same issue on multiple 7.15.0 sites (upgrade from 7.13.2). I've noticed it's an issue with larger sites (2000+ nodes).

Adding this in the web.config made the preview modus working again (like @bergmania mentioned):
<add key="Umbraco.Preview.Mode" value="Legacy" />

[mariojsnunes]

Yeah I can confirm. It works with that config.
Is this documented anywhere?
Thanks for the help!

[Shazwazza]

Both modes should work, if they do not then there is a bug. The Legacy mode is called "Legacy" for a reason and ideally it is not used anymore. Looks like there is a PR for this so will review.