Can't spawn custom mobs using procedures

Status
Works as designed
Issue description

So I'm working on a huge mod (the elementals thing I mentioned in one of my tickets) and I made a boss mob. What I want is to have a block that spawns him when right-clicked by a player with a certain item in their inventory. I've already got the testing part done, and I have most of the script made. The problem is, I can't use the 'spawn at x: y: z: entity:' to spawn a custom mob. When I take the bock out of the 'block inventory,' the first thing it says in the blank is 'mcreator_fireElemental.EntityfireElemental' (which is one of the other mobs in this mod). However, when I click the entity space, the dropdown menu thing opens as normal, but neither the fire elemental nor the chaos elemental (the mob I want to spawn) is in this list. Even though the "default" entry (the one it shows before I actually pick one) is 'mcreator_fireElemental.EntityfireElemental', only entities that are already in Minecraft as default mobs show up.

Issue comments

The entity selector just got out of the view. Move the block a bit lower so the whole drop-down is visible and you will be able to select your entities as they are no the top of the list which is cut off.