To expand on this, it also will occasionally freeze after converters are used after Bonestorm. This is a common thread here is that this is very similar to what was happening with Kruarg, Winged Bison, and Fel'Dras' summons when they fulfill their summon condition but don't have a slot (no target available frowny face appears, some time later, softlock). I bet if I checked, it'd be still happening with them, too.
I'm not sure why why Necrezza would even have the "no target available" popup her summon is unconditional. Maybe the no target available refers to dispel? Still, I'm reasonably certain this is the avenue that needs to be investigated.