[MC-125051] Incorrect structure names in advancements cause java.lang.NullPointerException

The bug

The advancement condition feature causes a crash when specifiying an incorrect structure name:

Example criteria

"criteria": {
    "find_structure": {
        "trigger": "minecraft:location",
        "conditions": {
            "feature": "incorrect_structure_name"
        }
    }
}

Steps to reproduce

Put the attached data pack in your world
Load up the world
Notice that it crashes immediately

Crash report

18w06a: crash-2018-02-10_21.05.48-server.txt

Description: Ticking player

java.lang.NullPointerException: Ticking player
	at bjo.a(SourceFile:167)
	at au.a(SourceFile:56)
	at au.a(SourceFile:36)
	at av$b.a(SourceFile:79)
	at av$a.a(SourceFile:106)
	at av.a(SourceFile:66)
	at sc.i(SourceFile:417)
	at sx.aa_(SourceFile:189)
	at hg.a(SourceFile:221)
	at sw.c(SourceFile:175)
	at net.minecraft.server.MinecraftServer.w(SourceFile:721)
	at net.minecraft.server.MinecraftServer.v(SourceFile:628)
	at cvl.v(SourceFile:155)
	at net.minecraft.server.MinecraftServer.run(SourceFile:533)
	at java.lang.Thread.run(Thread.java:745)