I have error in my python code(while producing a blender addon) which i couldn't figure it out. Can anyone please help me out? So, recently I was planning on creating a 3D model pack for blender 2.9 in which my 3d models can be easily accessible within blender through addon. My plan was to make a small little tool shelf (at the right panel) in which it shows preview image of the 3D model and when I click add button after selecting the desired 3D model from preview, it should append the object. 
As you can see it has categories and there are two categories and the previews are working fine.
My problem is:
When I click on Add button it suddenly pops up  with an error message which is given down below:

and I can't figure it out for a long time why it happens since I am a beginner in the field of coding.
I just reached till to this point by seeking help from others. I will provide the code of the addon so that if anyone could help me I would appreciate that. Thank you!
LINK OF CODE AND FILE STRUCTURE
This looks like a path error, similar to what is described in this question.
Append python script for Blender 2.8