• Minecraft: 1.12.2
• Forge: 2515
• Crafttweaker: 4.0.8

ICraftingInventory.getStack(integer) can return a null value, but you cannot test for a null value.

Consider the following snippet: (cInfo is an ICraftingInfo):

for i in 0 to cInfo.inventory.size {
	var item = cInfo.inventory.getStack(i);	
	if (item != null) {
		print(item.displayName);
	}
}

With the if conditional an "operator not supported" error occurs when the script is parsed.
Without the if conditional execution of the for loop silently aborts on trying to print the display name if getStack returns a null value (i.e. if the slot in the crafting inventory was empty).

So there isn't really a proper fix for this unfortunately, however I did add a isNull(<minecraft:dirt>) which should help you solve the issue :)

Thanks :)