Here are the steps in order to reproduce:
1. Create a new grade item in "Category and items"
2. Set the grade type to "Value"
3. Once it is created, modify the grade item and set the grade type to "None"
Result: the grade item disappears.
Is this normal ? My suggestion would be to remove the "None" after a new grade item is created. There is no need, in my opinion, for such an option to exist if the user can delete the grade item with the X icon in the "Actions" column.