[StackModifier] Yo Tell Developer about missing defaults! rustige_egg_f, 1
[StackModifier] Yo Tell Developer about missing defaults! horse.saddle.double, 1
[StackModifier] Yo Tell Developer about missing defaults! horse.saddle.single, 1

open plugin file StackModifier.cs then seach for this line {"horse.saddle", 1},

Underneath that add these 2 lines 
{"horse.saddle.single", 1},
{"horse.saddle.double", 1},

then search within the file for rustige_egg and underneath these entries 
{"rustige_egg_a", 1},
{"rustige_egg_b", 1},
{"rustige_egg_c", 1},
{"rustige_egg_d", 1},
{"rustige_egg_e", 1},

Add the following {"rustige_egg_f", 1},

SAVE and o.load Stackmodifier all fixed