// and add on every list like this

private List<Category> _SubChild_categories = new List<Category>();
public List<Category> SubChild_Categories
{
    get { return _SubChild_categories; }
    set { _SubChild_categories = value;
        OnPropertyChanged();
    }
}

private List<Category> _child_categories = new List<Category>();
public List<Category> Child_Categories
{
    get { return _child_categories; }
    set { _child_categories = value;
        OnPropertyChanged();
    }
}

// and add on every list like this

private List<Category> _SubChild_categories = new List<Category>();
public List<Category> SubChild_Categories
{
    get { return _SubChild_categories; }
    set { _SubChild_categories = value;
        OnPropertyChanged();
    }
}

private List<Category> _child_categories = new List<Category>();
public List<Category> Child_Categories
{
    get { return _child_categories; }
    set { _child_categories = value;
        OnPropertyChanged();
    }
}

// When you create the attributes, try to make them all according to this model (search the internet why)

private int? _category_Id;
public int? CategoryId
{
    get { return _category_Id; }
    set { _category_Id = value;
        OnPropertyChanged();
    }
}

// and here u can delete:
[You can delete from Category.cs what is (Virtual): It is used to overwrite a method from one class to another (override) and it is not the case for you (search the internet for what it is used for). Look to see what polymorphism means or what it is used for on chatgpt].

// When you create the attributes, try to make them all according to this model (search the internet why)

private int? _category_Id;
public int? CategoryId
{
    get { return _category_Id; }
    set { _category_Id = value;
        OnPropertyChanged();
    }
}

// and here u can delete:
[You can delete from Category.cs what is (Virtual): It is used to overwrite a method from one class to another (override) and it is not the case for you (search the internet for what it is used for). Look to see what polymorphism means or what it is used for on chatgpt].