About this issue, it is due to missing $album_all / $album_max in a script you have in the Picard 2 config, probably provided by a plugin (from Determining whether condition applies to multiple files - #5 by outsidecontext) which isn’t available (yet) on Picard 3, but it reveals a robustness issue: it should not prevent display of the metadata box.
I have a fix for the robustness issue, but not for the script or plugin.