Not sure about your icon problem but your error comes from the CommonProxy.getServerGuiElement()
TileEntityBrickFurnace tileEntity = (TileEntityBrickFurnace)world.getBlockTileEntity(x, y, z);
TileEntityStoneFurnace tileEntity1 = (TileEntityStoneFurnace)world.getBlockTileEntity(x, y, z);
if(tileEntity != null) {
...
}
This is not a valid way to check an object type. Use instanceof instead.
TileEntity tileEntity = world.getBlockTileEntity(x, y, z);
if (tileEntity instanceof TileEntityBrickFurnace) {
((TileEntityBrickFurnace) tileEntity).doSomething();
...
}
else if (tileEntity instanceof TileEntityStoneFurnace) {
...
}