This is my code

@Override
    public void writeToNBT(NBTTagCompound nbt) {
        super.writeToNBT(nbt);

        NBTTagList list = new NBTTagList();
        for (int i = 0; i < this.getSizeInventory(); ++i) {
            if (this.getStackInSlot(i) != null) {
                System.out.println(i);
                NBTTagCompound stackTag = new NBTTagCompound();
                stackTag.setByte("Slot", (byte) i);
                this.getStackInSlot(i).writeToNBT(stackTag);
                list.appendTag(stackTag);
            }
        }
        nbt.setTag("Items", list);

        if (this.hasCustomName()) {
            nbt.setString("CustomName", this.getCustomName());
        }
    }

 

The line super.writeToNBT(nbt); gives me this error

 

[23:21:52] [server thread/ERROR]: A TileEntity type robmart.rpgmode.entity.tileentity.ContainerTileEntity has throw an exception trying to write state. It will not persist. Report this to the mod author
java.lang.RuntimeException: class robmart.rpgmode.entity.tileentity.ContainerTileEntity is missing a mapping! This is a bug!
at net.minecraft.tileentity.TileEntity.writeToNBT(TileEntity.java:86) ~[TileEntity.class:?]
at robmart.rpgmode.entity.tileentity.ContainerTileEntity.writeToNBT(ContainerTileEntity.java:200) ~[ContainerTileEntity.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.writeChunkToNBT(AnvilChunkLoader.java:393) [AnvilChunkLoader.class:?]
at net.minecraft.world.chunk.storage.AnvilChunkLoader.saveChunk(AnvilChunkLoader.java:172) [AnvilChunkLoader.class:?]
at net.minecraft.world.gen.ChunkProviderServer.saveChunkData(ChunkProviderServer.java:257) [ChunkProviderServer.class:?]
at net.minecraft.world.gen.ChunkProviderServer.saveChunks(ChunkProviderServer.java:324) [ChunkProviderServer.class:?]
at net.minecraft.world.WorldServer.saveAllChunks(WorldServer.java:945) [WorldServer.class:?]
at net.minecraft.server.MinecraftServer.saveAllWorlds(MinecraftServer.java:427) [MinecraftServer.class:?]
at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:144) [integratedServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:548) [MinecraftServer.class:?]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_51]

I do register the tile entity, but my tile entity extends BlockContainer so i don't use registerTileEntity

 

EDIT: The tile entity works like normal but when saving the world the error is printed in the log