[1.12.2] [Unsolved] Custom GUI Won't Open

Hi, 

I have a block that has a Custom Gui, but every time I right-click the block it won't appear. I know that the TileEntities are working because I wrote a debug line that "checks" if it's working. This is the error that's shown in the console when I right-click:

[06:37:34] [Server thread/FATAL] [minecraft/MinecraftServer]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException

This is the onBlockActivated boolean in my Block:

    //BLOCK RIGHT-CLICKED (Opens GUI and Debug)
    @Override
    public boolean onBlockActivated(World world, BlockPos pos, IBlockState state, EntityPlayer player, EnumHand hand, EnumFacing side, float hitX, float hitY, float hitZ) {
        if (!world.isRemote) {
            //Sends a message in chat stating "Grinder Clicked"
            player.sendMessage(new TextComponentString("Grinder Clicked"));
            //Opens GUI
            player.openGui(Main.instance, ModGuiHandler.GRINDER, world, pos.getX(), pos.getY(), pos.getZ());
        }
        return true;
    }

I've also tried putting player.openGui outside of the if function, but that results in a crash. The ModGuiHandler is registered correctly as well, for NetworkRegistry.INSTANCE.

Additional Info:
I followed ShadowFacts tutorial for onBlockActivated

Any help is appreciated, thank you.