After the dragon is killed, the portal fails to open and immediately crashes the game. The Launcher gives this message:
The game crashed whilst ticking entity
Error: net.minecraftforge.fml.common.ObfuscationReflectionHelper$UnableToFindFieldException: java.lang.NoSuchFieldException: gateways
Exit Code: -1
Relaunching the game and opening the world again results in the same issue.
I am in 1.16.5 Forge

Duplicate of #211