[MCM-482] CRASH on Transforming an Entity While Riding It

Verification Build: 1.7, 1.8.0.24, 1.9.0.2 Beta

Partner Rank: 3

Summary:
If you are riding an entity that then triggers a transformation component via a behavior pack, the game will crash.

Our use case: In order to hide the fact that we are deleting an entity (we can't kill it because of the animation) and in order to maintain multiplayer positions, we set up a behavior pack where if the horse receives fatal damage via /effect, it will transform into an Area Effect Cloud which will then die 1 tick later. (NOTE: This bug is reproducible even without the behavior of the area effect cloud)

Impact:

Game Crash. I thought I had found a workaround but it only worked in singleplayer. There is no way to make the horse disappear reliably (even teleporting it into the void causes issues such as sometimes it doesn't die).

Repro Steps:

**First, observe the behavior without the player on the horse.

Download the provided behavior pack and equip it on a new world.
/summon horse (do not get on the horse)
/effect @e[type=horse] instant_damage 100 100 true
Observe the horse properly transforming into a sheep. This is the correct and expected behavior.
Continue with the repro steps below to see the issue.

-Singleplayer

1. Download the provided behavior pack and equip it on a new world.

2. /summon horse

3. Get on the horse.

4. /effect @e[type=horse] instant_damage 100 100 true

5. Observe the game crash.

-Multiplayer

**1. Download the provided behavior pack and equip it on a new world.

2. /summon horse

3. Get on the horse

4. Get another player (or more) to get on the horse at the same time (there are 7 seats in this pack)

5. /effect @e[type=horse] instant_damage 100 100 true.

6. Observe the game crash.

Observed Results:

The game hangs for a moment and then crashes and quits.

Expected Results:
Horse transforms into a sheep and all players are booted from riding the entity since the sheep has no riding component.

Screenshots/Videos attached: