[MC-123645] 17w49a+ crash: Initializing game / java.lang.IllegalStateException: Not a JSON Object: null

The bug

Minecraft crashes when launching snapshot versions 17w49a/b and 17w50a. I have already seen ~~MC-121738~~ , which says that disabling custom resolution would fix the problem. This didn't work for me, however.
To clarify, all versions and snapshots prior to 17w49a work perfectly fine, even 17w48a works perfectly.

Crash report

17w50a: crash-2017-12-28_17.59.14-client.txt

Description: Initializing game

java.lang.IllegalStateException: Not a JSON Object: null
	at com.google.gson.JsonElement.getAsJsonObject(JsonElement.java:90)
	at rl.a(SourceFile:73)
	at rl.a(SourceFile:66)
	at sd.a(SourceFile:35)
	at ry.a(SourceFile:38)
	at sa.a(SourceFile:33)
	at bnh.a(SourceFile:1170)
	at bne.an(SourceFile:437)
	at bne.a(SourceFile:365)
	at net.minecraft.client.main.Main.main(SourceFile:140)