Entering the nether portal crashes my game

^{Update by [Mojang] Mega_Spud (Jay Wells):}
Summary:
Travelling to the nether dimension causes the game to crash. Copy of the game save has been provided here.

Steps to Reproduce:

1. Import the above world save
2. Travel through a nether portal

Observed Results:
The game gets stuck on the portal / loading screen, and the game freezes and has to be closed, or crashes. Sometimes, the mobs that are in the nether can be heard (Villager, Cat, Phantom).

Expected Results:
The game should not freeze or crash when travelling to the nether.

Screenshots/Videos attached: Yes

Notes:
Workaround:

1. Load the world linked above
2. Teleport 5000 blocks away (/tp ~ ~ ~5000)
3. Build a nether portal, and travel through it to the nether
4. Set the difficulty to Peaceful
5. Fly/travel to coordinates 50 70 125 (Teleporting will likely cause the game to crash)
6. At the nether portal near 50 70 125, there are a group of mobs, including a Phantom, which has not despawned when the game was set to Peaceful
7. Break the nether portal and kill the Phantom mob

Following the steps in that order seems to allow the world to work properly, and travel between dimensions again.

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Original Description:
Every single time I enter any nether portal my game crashes. Very annoying since I have played on this world for months.