Let's hold an arrow in the main hand, and a crossbow in the off-hand. Make sure that there is only one arrow in your inventory.

Then, charge your crossbow, and drop your arrow while charging.

When charging is done, the server crashes.

There is a crash report.

==========

[Server thread/ERROR]: Encountered an unexpected exception
java.lang.AssertionError: TRAP
at net.minecraft.server.v1_14_R1.ItemStack.checkEmpty(ItemStack.java:82) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemStack.setCount(ItemStack.java:851) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemStack.add(ItemStack.java:855) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemStack.subtract(ItemStack.java:859) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemStack.cloneAndSubtract(ItemStack.java:128) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemCrossbow.a(ItemCrossbow.java:106) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemCrossbow.a(ItemCrossbow.java:96) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemCrossbow.a(ItemCrossbow.java:70) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.ItemStack.a(ItemStack.java:529) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.EntityLiving.clearActiveItem(EntityLiving.java:2860) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.PlayerConnection.a(PlayerConnection.java:1168) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.PacketPlayInBlockDig.a(SourceFile:40) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.PacketPlayInBlockDig.a(SourceFile:10) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.PlayerConnectionUtils.lambda$0(PlayerConnectionUtils.java:13) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.TickTask.run(SourceFile:18) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.IAsyncTaskHandler.executeTask(SourceFile:135) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.IAsyncTaskHandlerReentrant.executeTask(SourceFile:23) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.IAsyncTaskHandler.executeNext(SourceFile:114) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.MinecraftServer.aW(MinecraftServer.java:896) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.MinecraftServer.executeNext(MinecraftServer.java:889) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.IAsyncTaskHandler.executeAll(SourceFile:99) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.MinecraftServer.sleepForTick(MinecraftServer.java:872) ~[spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at net.minecraft.server.v1_14_R1.MinecraftServer.run(MinecraftServer.java:806) [spigot-1.14.jar:git-Spigot-1eece4f-7861651]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]