Player 2 kicked from server when using a player variable keybind

[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 it be? The keybind checks for an item in hand, and then sets a player persistent variable. So weird that this only happens to player 2.