private void LoadProducts()
{
    try
    {
        db = new Db();
        var gridvResult = (from u in db.Products
                           join c in db.Categories on u.CatId equals c.CategoryId
                           select new
                           {
                               PrId = u.ProductId,
                               ProductName =  u.ProductName.Substring(5), // Here I get error
                               CategoryName = c.CategoryName,
                           }).ToList();

        if (gridvResult != null)
        {
            dgvProduct.DataSource = null;
            dgvProduct.DataSource = gridvResult;
            dgvProduct.Columns[0].Visible = false;
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
        Exception inner = ex.InnerException;
        while (inner != null)
        {
            MessageBox.Show(inner.Message);
            inner = inner.InnerException;
        }
    }
}