When trying to import a series of test models exported from Blender 2.8 as FBX, I am getting the following error:

ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index

MAST_PrepModels.CreatePrefabsFromModels () (at Assets/FSP/MAST/Scripts/Tools/MAST_PrepModels.cs:185)
MAST_Tools_Interface.OnGUI () (at Assets/FSP/MAST/Scripts/Interface/MAST_Tools_Interface.cs:61)

If anyone else is seeing this issue, importing the same models as OBJ (along with their exported mat files) works perfectly without any issues at all.

I will address this over the next few days.  In the meantime, export from Blender as .obj and copy .mtl and .png with the models and it will work.

I cannot reproduce your error.  I've successfully imported FBX files with and without textures.  I tested both ways with models exported from Unity as well as Blender.

I would appreciate it if you could email me at fertilesoilproductions@gmail.com with the model that produces the error.