One player is kicked out of server when they left click

[Server thread/INFO] [net.minecraft.server.network.ServerGamePacketListenerImpl/]: ballguy15 lost connection: Internal Exception: java.lang.NullPointerException: Cannot invoke "net.minecraft.nbt.CompoundTag.getDouble(String)" because "nbt" is null

I think it has something to do with player variables, but i'm not sure exactly what. What can this be?

EDIT: Yeah it happens when I use a keybind which checks for an item in hand, and then sets a player persistent variable. It's so weird that it only happens with player 2.