Hi Pierre,
You can also go to GitHub, there you can download a zip file of the complete project:
https://github.com/mendix/TreeViewAndGridView
If you unpack it, you have a folder named "test_5_21_4". If you open the TreeviewDemo.mpr file in this folder, it should work inmediately. In my case i converted it to 5.21.5 and ran the project without any problems and could play around in the demo app inmediately.
Kind Regards,