After a little more testing, it appears to occur when dragging any piece of equipment within power armor, regardless of whether or not the armor is equipped.

* Power armor is in my inventory. Open armor's inventory. Drag battery into armor. -> Crash
* Power armor is equipped. Open armor's inventory ...

I've been able to do this multiple times in a row, so hopefully it's easy to reproduce on your end. I am wearing modular armor and I'm going to replace it with power armor and move all of my modules over to it. Here's what I'm doing:

1.) Click the power armor in my inventory and place it in my ...