Made a new world in 18w20c
Flew around
Left world
Re-entered world
Put Polished Granite, Lava Bucket and Slime Egg in Inventory
That yielded this exception:
[0518/232703:INFO:GameCallbacks.cpp(178)] game/m (Client thread) info Loaded 0 advancements [0518/232728:INFO:GameCallbacks.cpp(178)] game/net.minecraft.server.MinecraftServer (Server thread) info Kumasasa has made the advancement [Hot Stuff] [0518/232728:INFO:GameCallbacks.cpp(178)] game/cix (Client thread) info [CHAT] Kumasasa has made the advancement [Hot Stuff] [0518/232728:INFO:GameCallbacks.cpp(178)] game/m (Client thread) info Loaded 7 advancements [0518/232730:INFO:GameCallbacks.cpp(178)] game/xm (Client thread) error Something went wrong upgrading! java.lang.IllegalArgumentException: Unknown type: hotbar at adp.a(SourceFile:78) at adp$$Lambda$291/2005537360.apply(Unknown Source) at java.util.HashMap.computeIfAbsent(HashMap.java:1118) at adp.b(SourceFile:77) at xk.a(SourceFile:26) at xm.a(SourceFile:36) at xl.a(SourceFile:14) at chp.b(SourceFile:45) at chp.a(SourceFile:70) at cmq.b(SourceFile:485) at cmq.mouseReleased(SourceFile:462) at chx.a(SourceFile:69) at chx$$Lambda$857/1847000239.run(Unknown Source) at clm.a(SourceFile:429) at chx.a(SourceFile:69) at chx$$Lambda$643/1913718808.invoke(Unknown Source) at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:23) at org.lwjgl.system.JNI.invokeV(Native Method) at org.lwjgl.glfw.GLFW.glfwPollEvents(GLFW.java:2520) at cif.a(SourceFile:284) at chv.c(SourceFile:845) at chv.a(SourceFile:381) at net.minecraft.client.main.Main.main(SourceFile:144) [0518/232749:INFO:GameCallbacks.cpp(178)] game/net.minecraft.server.MinecraftServer (Server thread) warn Can't keep up! Did the system time change, or is the server overloaded? Running 4023ms behind, skipping 80 tick(s) [0518/232802:INFO:GameCallbacks.cpp(178)] game/dib (Client thread) warn Unable to play empty soundEvent: minecraft:entity.small_slime.squish [0518/232802:INFO:GameCallbacks.cpp(178)] game/dib (Client thread) warn Unable to play empty soundEvent: minecraft:entity.small_slime.jump [0518/232806:INFO:GameCallbacks.cpp(178)] game/net.minecraft.server.MinecraftServer (Server thread) warn Can't keep up! Did the system time change, or is the server overloaded? Running 3372ms behind, skipping 67 tick(s) [0518/232819:INFO:GameCallbacks.cpp(178)] game/cix (Client thread) info [CHAT] Saved screenshot as 2018-05-18_23.28.18.png [0518/232834:INFO:GameCallbacks.cpp(178)] game/cix (Client thread) info [CHAT] Saved screenshot as 2018-05-18_23.28.34.png [0518/233356:INFO:GameCallbacks.cpp(178)] game/dhr (Server thread) info Saving and pausing game...
Can sadly not reproduce.
Linked issues
discovered while testing
relates to
Comments


How can this affect 18w21a, it literally says in the title that its 18w20c exlusive

Hm, let's think together about that fact:
At the time of creating the ticket, 18w20c was out.
Thus title consists of 18w20c
A few days later 18w21a came out
Bug still not fixed
Bug was affecting 18w21a
Title was not updated
I just got this error when creating a new superflat world in 18w21a and opening the saved hotbars tab, which contains a hotbar I created probably in 18w14b or earlier.
One saved hotbar contained:
sandstone
redstone block
impulse command block (with no NBT?)
18w21a