How to reproduce:
Please, follow this steps:
Create New World.
More world options.
World type: Superflat
Customise.
Remove ALL the layers.
Presets.
Write with CAPITAL LETTERS the name of a block, for example:
MINECRAFT:BEDROCK
.
The game will crash.
Here is the crash report:
---- Minecraft Crash Report ----
// Oops.Time: 28/06/18 21:54
Description: mouseClicked event handleri: Non [a-z0-9_.-] character in namespace of location: MINECRAFT:BEDROCK
at ox.<init>(SourceFile:35)
at ox.<init>(SourceFile:43)
at bbr.a(SourceFile:136)
at btb.a(SourceFile:256)
at btb.b(SourceFile:272)
at btb.a(SourceFile:337)
at chp.a(SourceFile:70)
at cid$1.a(SourceFile:170)
at cfq.mouseClicked(SourceFile:103)
at cgm.mouseClicked(SourceFile:46)
at cer.b(SourceFile:67)
at cer$$Lambda$883/1930186237.run(Unknown Source)
at cig.a(SourceFile:430)
at cer.a(SourceFile:67)
at cer$$Lambda$753/2002232191.invoke(Unknown Source)
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:3108)
at cez.a(SourceFile:317)
at cep.c(SourceFile:833)
at cep.a(SourceFile:373)
at net.minecraft.client.main.Main.main(SourceFile:143)
A detailed walkthrough of the error, its code path and all known details is as follows:
----------------------------------------------------------------------------------------- Head --
Thread: Client thread
Stacktrace:
at ox.<init>(SourceFile:35)
at ox.<init>(SourceFile:43)
at bbr.a(SourceFile:136)
at btb.a(SourceFile:256)
at btb.b(SourceFile:272)
at btb.a(SourceFile:337)
at chp.a(SourceFile:70)
at cid$1.a(SourceFile:170)
at cfq.mouseClicked(SourceFile:103)
at cgm.mouseClicked(SourceFile:46)
at cer.b(SourceFile:67)
at cer$$Lambda$883/1930186237.run(Unknown Source)-- Affected screen --
Details:
Screen name: cid
Stacktrace:
at cig.a(SourceFile:430)
at cer.a(SourceFile:67)
at cer$$Lambda$753/2002232191.invoke(Unknown Source)
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:3108)
at cez.a(SourceFile:317)
at cep.c(SourceFile:833)
at cep.a(SourceFile:373)
at net.minecraft.client.main.Main.main(SourceFile:143)-- System Details --
Details:
Minecraft Version: 1.13-pre5
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 214100176 bytes (204 MB) / 788529152 bytes (752 MB) up to 1073741824 bytes (1024 MB)
JVM Flags: 9 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -Xmx1G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=16M
Launched Version: 1.13-pre5
LWJGL: 3.1.6 build 14
OpenGL: GeForce GTX 750 Ti/PCIe/SSE2 GL version 4.6.0 NVIDIA 388.13, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported. Using VBOs: Yes
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs:
Current Language: English (United Kingdom)
Profiler Position: N/A (disabled)
CPU: 8x Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz
Attachments
Comments


Read the crash report:
Non [a-z0-9_.-] character in namespace of location: MINECRAFT:BEDROCK
Capital letters in this instance are not supported anymore.

Yes, the cause of this is quite clear, but the game shouldn't crash due to direct user input.

I see. I would presume the game should just ignore it.

Exactly, as @Kumasasa said the game shouldn't crash due to a direct user input. The game should act the same as MC-116835.