Crash when using malformed block states or biome ids for superflat preset

The bug

Using malformed block states or biome ids for the superflat presets crashes the game.

Crash report

19w12b\: [^crash-2019-03-24_19.34.55-client.txt]

Description: mouseClicked event handler

n: Non [a-z0-9/._-] character in path of location: minecraft:dirt,minecraft:grass_block
	at qq.<init>(SourceFile:38)
	at qq.<init>(SourceFile:43)
	at cef.b(SourceFile:372)
	at cxo.a(SourceFile:63)
	at cyf.b(SourceFile:167)
	at cve.onPress(SourceFile:20)
	at cve.mouseClicked(SourceFile:45)
	at cvz.mouseClicked(SourceFile:45)
	at cub.b(SourceFile:86)
	at cyi.wrapScreenError(SourceFile:450)
	at cub.a(SourceFile:86)
	at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:36)
	at org.lwjgl.system.JNI.invokeV(Native Method)
	at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout(GLFW.java:3129)
	at csu.c(SourceFile:292)
	at cua.b(SourceFile:1044)
	at cua.d(SourceFile:993)
	at cua.b(SourceFile:412)
	at net.minecraft.client.main.Main.main(SourceFile:154)

How to reproduce

Try to use the following superflat preset

minecraft:bedrock;minecraft:dirt,minecraft:grass_block;minecraft:plains;plains