Object of type 'System.DBNull' cannot be converted to type 'System.String'. When throwing an exception from property.

public partial class GridViewTestForm : Form
{
    public GridViewTestForm()
    {
        InitializeComponent();

        List<GridViewDataColumn> gridColumns = new List<GridViewDataColumn>();

        gridColumns.Add(new GridViewTextBoxColumn
        {
            DataType = typeof(string),
            TextAlignment = ContentAlignment.MiddleLeft,
            FieldName = nameof(ReportCategoryViewModel.CategoryName),
            HeaderText = "Category Name",
            AutoSizeMode = BestFitColumnMode.DisplayedCells,
        });

        gridColumns.Add(new GridViewTextBoxColumn
        {
            DataType = typeof(string),
            TextAlignment = ContentAlignment.MiddleLeft,
            FieldName = nameof(ReportCategoryViewModel.NumberOfReportsUsingCategory),
            HeaderText = "# Reports",
            ReadOnly = true,
            AutoSizeMode = BestFitColumnMode.DisplayedCells,
        });

        gridColumns.Add(new GridViewTextBoxColumn
        {
            DataType = typeof(string),
            TextAlignment = ContentAlignment.MiddleLeft,
            FieldName = nameof(ReportCategoryViewModel.Description),
            HeaderText = "Description",
        });

        // Add the columns to the grid
        radGridView1.MasterTemplate.Columns.AddRange(gridColumns);
        radGridView1.EnableSorting = true;
        radGridView1.MasterTemplate.AllowMultiColumnSorting = true;

        radGridView1.DataError += RadGridView1_DataError;
        radGridView1.CellValidating += RadGridView1_CellValidating;

        var viewModel = new ReportCategoryManagementViewModel();
        viewModel.LoadReportCategories();
        radGridView1.DataSource = viewModel.ReportCategories;
    }

    private void RadGridView1_CellValidating(object sender, CellValidatingEventArgs e)
    {
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = string.Empty;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.CustomizeBorder = false;
    }

    private void RadGridView1_DataError(object sender, GridViewDataErrorEventArgs e)
    {
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].ErrorText = e.Exception.Message;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.CustomizeBorder = true;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.DrawBorder = true;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.BorderWidth = 1;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.BorderGradientStyle = Telerik.WinControls.GradientStyles.Solid;
        radGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Style.BorderColor = Color.Red;
        e.Cancel = true;
    }
}
public class NotifyPropertyBase : INotifyPropertyChanged
{
    public bool IgnoreValidation { get; set; } = false;
    public event PropertyChangedEventHandler PropertyChanged;
    public virtual void OnPropertyChangedRaised([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}
public class ReportCategoryManagementViewModel : NotifyPropertyBase
{
    private ObservableCollection<ReportCategoryViewModel> _reportCategories = null;
    public ObservableCollection<ReportCategoryViewModel> ReportCategories => _reportCategories;
    public event EventHandler<List<ReportCategoryViewModel>> ReportCategoriesLoaded;
    public bool _isBusy = false;
    public bool IsBusy { get => _isBusy; set { if (_isBusy != value) { _isBusy = value; OnPropertyChangedRaised(); } } }
    public ReportCategoryManagementViewModel()
    {

    }
    public void LoadReportCategories()
    {
        _reportCategories = new ObservableCollection<ReportCategoryViewModel>()
        {
            new ReportCategoryViewModel()
            {
                IgnoreValidation = true,
                CategoryName = "My category",
                NumberOfReportsUsingCategory = 2,
                Description = "my optional description for my category"
            }
        };
        foreach(var category in _reportCategories)
            category.IgnoreValidation = false;
    }
}
public class ReportCategoryViewModel : NotifyPropertyBase
{
    public EntityState State { get; private set; } = EntityState.Unchanged;
    private string _categoryName = string.Empty;
    public string CategoryName
    {
        get => _categoryName;
        set
        {
            if (_categoryName != value)
            {
                _categoryName = value;
                OnPropertyChangedRaised();
                if (!IgnoreValidation)
                {
                    if (!string.IsNullOrWhiteSpace(_categoryName))
                    {
                        if (_categoryName.Length > 5)
                            throw new Exception("Maximum character is 5.");
                    }
                    else
                    {
                        _categoryName = string.Empty;
                        throw new Exception("Please enter category name.");
                    }
                }
            }
        }
    }
    private int _numberOfReportsUsingCategory;
    public int NumberOfReportsUsingCategory { get => _numberOfReportsUsingCategory; set { if (_numberOfReportsUsingCategory != value) { _numberOfReportsUsingCategory = value; OnPropertyChangedRaised(); } } }
    private string _description;
    public string Description { get => _description; set { if (_description != value) { _description = value; OnPropertyChangedRaised(); } } }
}