With the help of Marcolino I finally can enjoy the sight of the Trollino in the Modeleditor:-)
I had a lot of material files, which corrupted the modeleditor (I think)
I googled how to merge materials again and this time I found a file.
It's an add-on which is standard in Blender named: Materials Utils Special.
So I activated this file and then;
- Cleaned the material slots
- Merge based names (auto setting)
Naamloos4-10 (4).pngNaamloos4-10 (5).png
Hope this will work for the other models too.
Thanks for your help Marcolino!!!