Error message for player advancement data containing only "null" calls advancements "achievements"

The bug

The error message being logged for a player advancement data file containing only null calls advancements "achievements". This is the only place the string "achievement" exists in 17w18b.

The affected method is nd.g() in 17w18b.

17w18b

[12:53:33] [Server thread/ERROR]: Couldn't parse player advancements in ###
com.google.gson.JsonParseException: Found null for achievements
	at nd.g(SourceFile:118) [17w18b.jar:?]
	at nd.<init>(SourceFile:61) [17w18b.jar:?]
	at oy.h(SourceFile:918) [17w18b.jar:?]
	at od.<init>(SourceFile:176) [17w18b.jar:?]
	at oy.g(SourceFile:429) [17w18b.jar:?]
	at op.b(SourceFile:120) [17w18b.jar:?]
	at op.G_(SourceFile:66) [17w18b.jar:?]
	at go.a(SourceFile:232) [17w18b.jar:?]
	at om.c(SourceFile:187) [17w18b.jar:?]
	at net.minecraft.server.MinecraftServer.D(SourceFile:652) [17w18b.jar:?]
	at net.minecraft.server.MinecraftServer.C(SourceFile:562) [17w18b.jar:?]
	at cgl.C(SourceFile:154) [17w18b.jar:?]
	at net.minecraft.server.MinecraftServer.run(SourceFile:466) [17w18b.jar:?]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_131]

How to reproduce

1. Close any open Minecraft instances, otherwise the file might get overwritten

2. Edit a player advancement data file (advancements folder of a world folder) to contain only null

3. Start Minecraft and open the world