Item with NBT works in single player, crashes client when obtained on server

Hello friends,

I created an item with a GUI. 
The GUI sets an NBT text tag for the item.

In single player this works fine with no issues.
In multiplayer, the instant you craft the item (or select it from creative menu) the client immediately crashes with the following message:

Internal Exception: io.netty.handler.codec.DecoderException:
java.lang.NullPointerException: Cannot invoke "net.minecraft.nbt.CompoundTag.contains(String, int)" because "nbt" is null

Singleplayer:

Multiplayer:

Any help or advice would be appreciated.