The following is the command in question:

/summon leash_knot ~ ~ ~ {Passengers:[{id:Zombie}]}

DO NOT USE ON VALUED WORLD

After running this command once, the game will crash. When you re-enter the world and run the same command, the game will crash again but also delete your world (the level.dat file)

Unfortunately this happened to me on a valuable world and so I lost that world. It was also stupid of me not to make any back-ups, but it was only one of the first nights I was working on it, and I shouldn't have to rely on making back-ups.

I believe the problem is when the LeashKnot naturally despawns, the passengers 'pop-off' and cause issues. In both cases, the timing of the crash after spawning seems to be consistent with the lifetime of the LeashKnot.

java.lang.NullPointerException: Ticking entity
	at sa.A(SourceFile:1287)
	at sa.p(SourceFile:1834)
	at rr.aw(SourceFile:1498)
	at sa.aw(SourceFile:1840)
	at aht.a(SourceFile:1513)
	at lp.a(SourceFile:719)
	at aht.g(SourceFile:1493)
	at aht.a(SourceFile:1559)
	at lp.a(SourceFile:719)
	at aht.g(SourceFile:1493)
	at aht.k(SourceFile:1383)
	at lp.k(SourceFile:552)

Code analysis by marcono1234 can be found in this comment.