This will only work if the existing texturemaps are blocks(0) and items(1). But there are methods to register custom texture maps (IE does this for its revolvers), which leads to this event being fired again, and you trying to stitch your textures into my revolver sheet.

To fix this, just check for the sheet you want, rather than the ones you don't want,
if(event.map.getTextureType()!=1)
return;

123 not it!

Yeah, I knew about this back when you animated the revolvers, but I hadn't gotten around to fixing it yet. It's already a todo for me.