MediaWiki version: 1.35.0-wmf.28
PHP Notice: Undefined index: skin
This goes back to at least 1.35.0-wmf.24 at a fairly low volume, but I wasn't able to find an open task for it.
This seems to be occuring in the save_collection command which can be found on Special:Book with the following configuration and clicking "save book" in Save and share your book.
It is only triggered when you rewrite (e.g. hit save on a page that already exists)
wfLoadExtension('Collection'); $wgGroupPermissions['*']['collectionsaveasuserpage'] = true; $wgGroupPermissions['*']['collectionsaveascommunitypage'] = true;
The issue is getSkin requires a 'skin' value to be set on the SkinTemplate. This isn't happening. Likely introduced in January by @DannyS712 while working on T241616. Collection is a pain to test, so easy mistake to make.