The machine i am working with is this:
makeshift_smelter.txt
it is completely vanilla except the input/output port and the controller

The recipe and the only recipe i have is this:
makeshift_smelter_iron.txt

and when the crafting progress reach 99% the world crash and i checked the log, the error is:
error.txt

I will quote some lines that i think is important in the file for easier reading:

Line 2: net.minecraft.util.ReportedException: Ticking block entity
Line 8: Caused by: java.lang.IllegalStateException: Invalid item output!
Line 64-66:-- Block entity being ticked --
Details:
Name: minecraft:tilemachinecontroller // hellfirepvp.modularmachinery.common.tiles.TileMachineController

But the point is my output item is actually minecraft:iron_ingot it is not even a modded item, so why invalid?

I checked all the json file syntax, it is correct, i did make sure there is output hatches in my machine for it to output, so i am not sure why...