mods.gregtech.Assembler.addRecipe(<gregtech:gt.metaitem.01:1246>, <gregtech:gt.metaitem.01:1240>, null, <liquid:molten.rubber> * 144, 100, 8);
When an assembler recipe uses a single item in first slot, you must add an integrated circuit into second slot. null does not work here.
I don't know if it is a limitation in GTTweaker or in Gregtech API though.
Gregtech uses Integrated Circuit 24 with assembler cable coating.
This should work: