[MC-185004] [Crash] java.lang.NullPointerException - Crash on entering end/nether portal when dimension is removed in worldgen settings

The bug

Entering portal to the nether or end when that dimension is not present in the json causes the game to crash.

The crash report reports a null pointer exception ticking the player when attempting to travel to the nether and a null pointer exception colliding the player with a block when entering an end portal.

I would expect the game to either refuse to generate a portal or simply not transfer the player to the dimension in this situation.

How to reproduce

Download the worldgen settings worldgen_settings_export.json
Create a new world and select the downloaded file when importing world settings
Make a nether portal and enter it
The game crashes

Stack traces

20w21a: entering_end_crash_report.txt

Description: Colliding entity with block

java.lang.NullPointerException: Colliding entity with block
	at yx.a(SourceFile:640)
	at bwk.a(SourceFile:44)
	at cei$a.a(SourceFile:864)
	at any.at(SourceFile:955)
	at any.a(SourceFile:644)
	at aoj.a(SourceFile:2068)
	at aoj.e(SourceFile:2025)
	at bdi.e(SourceFile:1573)
	at aoj.l(SourceFile:2398)
	at bdi.l(SourceFile:579)
	at aoj.h(SourceFile:2208)
	at bdi.h(SourceFile:275)
	at yx.i(SourceFile:417)
	at zo.b(SourceFile:201)
	at me.a(SourceFile:234)
	at zn.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:905)
	at net.minecraft.server.MinecraftServer.a(SourceFile:832)
	at enr.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:691)
	at java.lang.Thread.run(Thread.java:745)

20w21a: entering_nether_crash_report.txt

Description: Ticking player

java.lang.NullPointerException: Ticking player
	at yx.a(SourceFile:640)
	at any.bf(SourceFile:1885)
	at any.ad(SourceFile:438)
	at aoj.ad(SourceFile:312)
	at any.h(SourceFile:420)
	at aoj.h(SourceFile:2132)
	at bdi.h(SourceFile:275)
	at yx.i(SourceFile:417)
	at zo.b(SourceFile:201)
	at me.a(SourceFile:234)
	at zn.c(SourceFile:171)
	at net.minecraft.server.MinecraftServer.b(SourceFile:905)
	at net.minecraft.server.MinecraftServer.a(SourceFile:832)
	at enr.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:691)
	at java.lang.Thread.run(Thread.java:745)

20w21a: sheep_entering_nether_crash_report.txt

Description: Ticking entity

java.lang.NullPointerException: Ticking entity
	at any.a(SourceFile:2292)
	at any.bf(SourceFile:1885)
	at any.ad(SourceFile:438)
	at aoj.ad(SourceFile:312)
	at aok.ad(SourceFile:262)
	at any.h(SourceFile:420)
	at aoj.h(SourceFile:2132)
	at aok.h(SourceFile:329)
	at yw.a(SourceFile:627)
	at bpa.a(SourceFile:591)
	at yw.a(SourceFile:420)
	at net.minecraft.server.MinecraftServer.b(SourceFile:893)
	at net.minecraft.server.MinecraftServer.a(SourceFile:832)
	at enr.a(SourceFile:89)
	at net.minecraft.server.MinecraftServer.run(SourceFile:691)
	at java.lang.Thread.run(Thread.java:745)