How to Store RichTextBox to SQLServer Database in WPF

Question

I'm a bit new to C# and WPF and experiencing a very frustrating issue storing a richtextbox with formatting to my database using entity framework.

Here is my code.

namespace simpleton
 public partial class AddNewOE : Window
   {
      //db connection
      simpletonDBEntities _db = new simpletonDBEntities();
      public AddNewOE()
      {
         InitializeComponent();
       }

       private void insertobBtn_Click(object sender, RoutedEventArgs e)
       {

           db_entry newdb_entries = new db_entry()
          {
             ReportDetails = ConvertRtbToBytes(rtfText)
           };
            _db.ob_entries.Add(newOb_entries);
           _db.SaveChanges();
        }
       public static byte[] ConvertRtbToBytes(string richTextBox)
       {
            byte[] data = null; string rtfText; //string to save to db
           TextRange tr = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);
           using (MemoryStream ms = new MemoryStream())
           {
                   tr.Save(ms, DataFormats.Rtf);
                   rtfText = Encoding.ASCII.GetString(ms.ToArray());
             }
            return data;
         }
     }

}

Please help me debug it, I'm getting the error on the ConvertRtbToBytes Function that... 'string' does not contain definition for 'Document' and no accessible extension method 'Document' accepting a first argument of type string could be found (are you missing a using directive or an assembly reference?)

I am also sure the general code is not Okay, any help will be massively appreciated.