I got a problem when adding a new feature : the polygon disappeared each time I right-clicked. This was cause by the fact that some features was missing : Because the countries of the layer were categorised with their GDP_MD_EST field, and that I didn't write anything in that field, the new polygon wasn't add to the layer.

Either you need to write something in the field used to categorised the layer, or you remove the categories. If I'm not mistaking, this isn't tell in the tutorial.

Anyway, thank you for you work. It's the first good example I found after hours of searches on this topic, and it's pretty well explained.