Cannot view dependent pages when the link exists on content outside of SiteTree

[ScopeyNZ]

Description

When viewing dependent pages (added by CMS) with Subsites installed the page will crash when the link to the page exists on a DBField that does not belong to the SiteTree (or a subclass of it).

Versions tested

CMS 4.4.x-dev
Subsites 2.3.x-dev
(Elemental 4.1.x-dev)

Steps to reproduce

• Install Elemental as an example of something that's not a page that contains content
• Create a blocks page and add a content block. Add a link to the HTML field provided by the content block and link to an existing page on the site. Save and publish.
• Browse to edit the page in the CMS that you linked to in the previous step.
• Click the "Dependent Pages" tab that should appear
• Observe an error in an alert and that the tab does not load

Running composer remove silverstripe/subsites, dev/build, and then reloading the page resolves the issue.

Notes

From a very brief look, it seems as though there's some code somewhere assuming the existence of a Subsite attribute on whatever the end (Parent) relation is on a SiteTreeLink (I think). This is an invalid assumption.

[michalkleiner]

Still an issue at the end of 2021.

[mikey-harveycameron]

Still an issue. If stuck, write an extension to sitetree and use updateCMSFields to unset 'Subsite.Title' from the gridfield's setDisplayFields.