Artisanal Reskins modify the sprites of the inserters. That mod is attempting to do the same, however without verifying what the current state of the inserter sprites are prior to doing so, which is why the error is occurring. Ideally, they would change how they're modifying graphics to either set the graphics they need on the inserter they're modifying in full rather than the piecemeal approach they're using.
The issue in detail: they assume that the regular inserter is using the vanilla sprites, copy it, and then surgically modify it. AR changes it to use layered sprites, so those fields do not exist, and accessing them in the way that lignumis does is an error.
