Using a superflat preset with an invalid biome causes a crash

When creating a new world with a custom superflat preset, if a preset with an invalid biome ID (for example "air;testing") is used, the game will accept it, and when exiting out of the customize screen, the game will show a "Saving world" screen and crash.

Below is the crash report:

---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 9/3/20, 12:15 AM
Description: mouseClicked event handler

java.lang.IllegalStateException: Missing: ResourceKey[minecraft:worldgen/biome / minecraft:testing]
	at gm.d(SourceFile:378)
	at don.b(SourceFile:276)
	at cpc.e(SourceFile:188)
	at cpc.c(SourceFile:131)
	at chi.<init>(SourceFile:26)
	at dsd.a(SourceFile:105)
	at dns.b(SourceFile:88)
	at dlg.b(SourceFile:33)
	at dla.a(SourceFile:16)
	at dle.a(SourceFile:149)
	at dme.a(SourceFile:27)
	at djx.b(SourceFile:92)
	at doq.a(SourceFile:435)
	at djx.a(SourceFile:92)
	at djx.c(SourceFile:162)
	at any.execute(SourceFile:94)
	at djx.b(SourceFile:162)
	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:3174)
	at com.mojang.blaze3d.systems.RenderSystem.limitDisplayFPS(SourceFile:127)
	at djw.e(SourceFile:1046)
	at djw.e(SourceFile:656)
	at net.minecraft.client.main.Main.main(SourceFile:215)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
	at gm.d(SourceFile:378)
	at don.b(SourceFile:276)
	at cpc.e(SourceFile:188)
	at cpc.c(SourceFile:131)
	at chi.<init>(SourceFile:26)
	at dsd.a(SourceFile:105)
	at dns.b(SourceFile:88)
	at dlg.b(SourceFile:33)
	at dla.a(SourceFile:16)
	at dle.a(SourceFile:149)
	at dme.a(SourceFile:27)
	at djx.b(SourceFile:92)

-- Affected screen --
Details:
	Screen name: dns
Stacktrace:
	at doq.a(SourceFile:435)
	at djx.a(SourceFile:92)
	at djx.c(SourceFile:162)
	at any.execute(SourceFile:94)
	at djx.b(SourceFile:162)
	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:3174)
	at com.mojang.blaze3d.systems.RenderSystem.limitDisplayFPS(SourceFile:127)
	at djw.e(SourceFile:1046)
	at djw.e(SourceFile:656)
	at net.minecraft.client.main.Main.main(SourceFile:215)

-- System Details --
Details:
	Minecraft Version: 1.16.2
	Minecraft Version ID: 1.16.2
	Operating System: Linux (amd64) version 5.8.4-artix1-1
	Java Version: 14.0.2, N/A
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 354367928 bytes (337 MB) / 1073741824 bytes (1024 MB) up to 2147483648 bytes (2048 MB)
	CPUs: 4
	JVM Flags: 8 total; -Xss1M -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
	Launched Version: 1.16.2
	Backend library: LWJGL version 3.2.2 build 10
	Backend API: Mesa DRI Intel(R) HD Graphics 4000 (IVB GT2) GL version 3.0 Mesa 20.1.6, Intel Open Source Technology Center
	GL Caps: Using framebuffer using OpenGL 3.0
	Using VBOs: Yes
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Graphics mode: fast
	Resource Packs: vanilla, programer_art
	Current Language: English (US)
	CPU: 4x Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz