The bug
I ran this command and crashed out to the launcher (from this page):
/scoreboard objectives add clicked minecraft.used:minecraft.carrot_on_a_stick
After loading the world back up I was unable to recreate the issue.
Stack trace
21w20a\: [^crash-2021-05-23_16.12.41-client.txt]
Description: keyPressed event handler
java.lang.NullPointerException: Cannot invoke "String.toLowerCase(java.util.Locale)" because "☃" is null
at di.b(SourceFile:190)
at dz.listSuggestions(SourceFile:61)
at com.mojang.brigadier.tree.ArgumentCommandNode.listSuggestions(ArgumentCommandNode.java:69)
at com.mojang.brigadier.CommandDispatcher.getCompletionSuggestions(CommandDispatcher.java:598)
at dwm.a(SourceFile:184)
at dyw.b(SourceFile:77)
at dwr.d(SourceFile:152)
at dwr.b(SourceFile:147)
at dwr.a(SourceFile:290)
at dxg.a(SourceFile:64)
at dzz.a(SourceFile:123)
at dyw.a(SourceFile:87)
at duw.a(SourceFile:367)
at dzz.a(SourceFile:489)
at duw.a(SourceFile:363)
at duw.c(SourceFile:455)
at aqe.execute(SourceFile:102)
at duw.b(SourceFile:455)
at org.lwjgl.glfw.GLFWKeyCallbackI.callback(GLFWKeyCallbackI.java:37)
at org.lwjgl.system.JNI.invokeV(Native Method)
at org.lwjgl.glfw.GLFW.glfwPollEvents(GLFW.java:3101)
at com.mojang.blaze3d.systems.RenderSystem.flipFrame(SourceFile:165)
at dpa.e(SourceFile:310)
at duy.f(SourceFile:1130)
at duy.e(SourceFile:721)
at net.minecraft.client.main.Main.main(SourceFile:217)
Can confirm that this is an issue. This did not appear to happen in 1.16.5 or 21w19a.