How do I read the NBT Tag Data of a map on a server?

[TheHook]

As the title suggests, I am trying to gather NBT Tag data from a map that is in my inventory on a server. The map has an image on it that I am trying to convert into an external Base64 format, or PNG file. But that part is not really important. The main question is given a filled map item on a server, how would I acquire the information I need? Some starter code that I tried returned java.lang.NullPointerException obviously because maps don't store their data in the item, so the keys that I'm using as arguments don't exist.

 

minecraft.player.inventory.getStackInSlot(0).getTagCompund().getCompoundTag("data").getByteArray("colors");

 

I attached the NBT hierarchy of a map item if that helps.

[TheHook]

I forgot to mention that this project is being in in version 1.12.2 of Minecraft. I'm not too sure how to use the class, how would I construct it? What is "mapName" in the game?

Edited October 26, 2020 by TheHook
more precise question