Thread:Extension talk:User Merge and Delete/"Fatal error: Call to undefined method Xml::hidden()/reply

This was already fixed last year (see r87595). Maybe you're using an old version of the extension with a newer version of MediaWiki?

Make sure you are using matching versions (e.g. MediaWiki 1.17, with the UserMerge extension from the 1.17 branch etc.).

was removed in MediaWiki 1.18.