Crash when a parent item is dragged into its list of children

[ravenfish]

When attempting to drag advantages into a parent item, I accidentally clicked the parent item itself and dragged it into the middle of the list of child advantages. The program froze for a few seconds and then crashed. I presume trying to make a trait the parent of itself led to an infinite loop. Setting the program to refuse to make a trait its own child, rather than trying to do so and crashing, might be worth doing.