This is the code

 

package com.sixonethree.randomutilities.block.tile;

import java.util.List;
import java.util.UUID;

import com.sixonethree.randomutilities.init.ModBlocks;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.entity.player.EntityPlayerMP;
import net.minecraft.inventory.IInventory;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.tileentity.TileEntity;
import cpw.mods.fml.common.FMLCommonHandler;

public class TileEntityMagicChest extends TileEntity implements IInventory {

private String owner;
private int facing;
private ItemStack stack;

public TileEntityMagicChest() {
	super();
	this.owner = "";
}

@Override public boolean canUpdate() { return true; }

@SuppressWarnings("unchecked") @Override public void updateEntity() {
	if (this.owner.isEmpty() || this.owner.equalsIgnoreCase("")) return;
	if (!this.worldObj.isRemote) {
		List<EntityPlayerMP> players = FMLCommonHandler.instance().getMinecraftServerInstance().getConfigurationManager().playerEntityList;
		for (EntityPlayerMP player : players) {
			if (player.getUniqueID().equals(UUID.fromString(this.owner))) {
				if (this.stack != null) {
					stack.getItem().onUpdate(stack, this.worldObj, player, 0, false);
				}
				break;
			}
		}
	}
}

public void setOwner(String owner) {
	this.owner = owner;
	markDirty();
}

@Override public void readFromNBT(NBTTagCompound compound) {
	super.readFromNBT(compound);
	this.owner = compound.hasKey("owner") ? compound.getString("owner") : "";
}

@Override public void writeToNBT(NBTTagCompound compound) {
	super.writeToNBT(compound);
	compound.setString("owner", owner);
}

public int getFacing() { return this.facing; }

public void setFacing(int facing2) {
	this.facing = facing2;
}

/* IInventory */

@Override public int getSizeInventory() { return 1; }

@Override public ItemStack getStackInSlot(int slot) {
	if (slot != 0) return null;
	return this.stack;
}

@Override public ItemStack decrStackSize(int slot, int amount) {
	if (this.stack != null) {
		this.stack.stackSize -= amount;
		if (this.stack.stackSize <= 0) this.stack = null;
		markDirty();
	}
	return this.stack;
}

@Override public ItemStack getStackInSlotOnClosing(int slot) {
	return slot == 0 ? this.stack : null;
}

@Override public void setInventorySlotContents(int slot, ItemStack content) {
	if (slot == 0) this.stack = content;
}

@Override public String getInventoryName() { return "Magic Chest"; }
@Override public boolean isUseableByPlayer(EntityPlayer player) {
	if (worldObj == null) { return true; }
	if (worldObj.getTileEntity(xCoord, yCoord, zCoord) != this) { return false; }
	if (player.getDistanceSq((double) xCoord + 0.5D, (double) yCoord + 0.5D, (double) zCoord + 0.5D) <= 64D) return player.getUniqueID().equals(UUID.fromString(this.owner));
	return false;
}

@Override public void openChest() {
	if (worldObj == null) return;
	worldObj.addBlockEvent(xCoord, yCoord, zCoord, ModBlocks.magicChest, 1, 1);
}

@Override public void closeChest() {
	if (worldObj == null) return;
	worldObj.addBlockEvent(xCoord, yCoord, zCoord, ModBlocks.magicChest, 1, 0);
}

@Override public boolean isCustomInventoryName() {
	return false;
}

@Override public int getInventoryStackLimit() {
	return 64;
}

@Override public boolean isItemValidForSlot(int index, ItemStack stack) {
	return true;
}
}

 

This is the error

 

java.lang.RuntimeException: class com.sixonethree.randomutilities.block.tile.TileEntityMagicChest is missing a mapping! This is a bug!
at net.minecraft.tileentity.TileEntity.writeToNBT(TileEntity.java:96) ~[TileEntity.class:?]
at com.sixonethree.randomutilities.block.tile.TileEntityMagicChest.writeToNBT(TileEntityMagicChest.java:55) ~[TileEntityMagicChest.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.writeChunkToNBT(AnvilChunkLoader.java:395) [AnvilChunkLoader.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.saveChunk(AnvilChunkLoader.java:204) [AnvilChunkLoader.class:?]
at net.minecraft.world.gen.ChunkProviderServer.saveChunkData(ChunkProviderServer.java:276) [ChunkProviderServer.class:?]
at net.minecraft.world.gen.ChunkProviderServer.saveChunks(ChunkProviderServer.java:329) [ChunkProviderServer.class:?]
at net.minecraft.world.WorldServer.saveAllChunks(WorldServer.java:863) [WorldServer.class:?]
at net.minecraft.server.MinecraftServer.saveAllWorlds(MinecraftServer.java:374) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:640) [MinecraftServer.class:?]
at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:118) [integratedServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:489) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:756) [MinecraftServer$2.class:?]

You need to register your TileEntity using

GameRegistry.registerTileEntity

.

Derpity derp, I did it before and forgot, I'm dumb.