Help with "nbt" is null problem

I'm creating a mod for my friends that adds two dimensions for them and the output from them is specific. Actually, in the second dimension you create a console and you need to enter a couple of commands. In a single player game everything is stable, but if you check the performance on a server with two players, only the second one crashes with the error

• [16:25:59] [Server thread/INFO] [minecraft/ServerGamePacketListenerImpl]: ProGaMEr11052 lost connection: Internal Exception: java.lang.NullPointerException: Cannot invoke "net.minecraft.nbt.CompoundTag.getBoolean(String)" because "nbt" is null 

I've been trying to overcome this problem for the second day now because it occurs in other mechanics that I added. So here's the procedure

This happend in GUI. GUI is binded to a block