SchallMachineScaling depends on steam-engine having the icon property, and your mod sets it to nil. I posted to their Discussion section. Could you load your mod after theirs?

https://mods.factorio.com/mod/SchallMachineScaling/discussion/5da04dcec9d053000d1a9e1e
From Schallfalke:

"PS1: If property icons is not nil, the game automatically take that as higher preference in icon display. So it is UNNECESSARY to remove property icon (set to nil)."

After learning to mod, I found the problem IS with your mod: you assume an thing.icon exists. This is ironic because YOUR items and entities do not have an icon.

To fix it:

First:
local base_icon_path
local base_icons_path
local icon_t

Then above the tier1 to tiermax and tech_icon_path:
base_icon_path = data.raw.item[machine_name].icon
base_icons_path = data.raw.item[machine_name].icons

Above the data:extend in the loop:

  icon_t = {}
  if base_icon_path then
    icon_t[1] = { icon = base_icon_path }
    icon_t[2] = { icon = overtier_icon_prefix..tier..overtier_icon_suffix, tint = overtier_icon_tint }
  else
    icon_t[1] = { icon = base_icons_path[1].icon }
    icon_t[2] = { icon = overtier_icon_prefix..tier..overtier_icon_suffix, tint = overtier_icon_tint }
  end

And then in the item and entity:
icons = icon_t,

I did those in steam-engines and steam-turbines. It might be worth generalizing it, and doing it for each group, since technically one of icon+icon_size, icons[1-n].icon + icon_size, or icons[1-n].icon + icons[1-n].icon_size are required.

Link to files with changes:
https://drive.google.com/open?id=1U8Vf1DJlYQXaLh9CHGZJE9gwoX4xsMhZ