RibbonComboBox- data selection

I have a ribboncombobox and it bind with a table. As I want student ID will display in the combobox, But when I select a student Id from the ribboncombobox it will give following error.

Can someone please help me.

Error is-

An unhandled exception of type 'System.InvalidCastException' occurred in RibbonControlsLibrary.dll

Additional information: Unable to cast object of type 'System.Data.DataRowView' to type 'Microsoft.Windows.Controls.Ribbon.RibbonComboBoxItem'.

<r:RibbonComboBox Name="childnamecombo" ItemsSource="{Binding}"   SelectedValuePath="stid"   Margin="34,56,0,0" VerticalAlignment="Top" HorizontalAlignment="Left" Width="168" IsEditable="True" IsReadOnly="True" IsDropDownOpen="false" SelectedIndex="-1" SelectionChanged="selectchildindividualobservation_SelectionChanged">

                <r:RibbonComboBox.ItemTemplate >

                <DataTemplate >

                    <TextBlock Text="{Binding stid}"/>

                DataTemplate>

            r:RibbonComboBox.ItemTemplate>

 r:RibbonComboBox>

SQLiteConnection SQLiteCon = new SQLiteConnection();

SQLiteCon.ConnectionString = @"data source=C:\student.s3db";

            SQLiteCommand cmd = new SQLiteCommand();

            cmd.Connection = SQLiteCon;

            cmd.CommandType = CommandType.Text;

            cmd.CommandText = "select * from studentrecord";

            SQLiteDataAdapter sqlitead = new SQLiteDataAdapter();

            sqlitead.SelectCommand = cmd;

            DataSet ds = new DataSet();

            try

            {

                sqlitead.Fill(ds, "studentrecord");

                 // binding data to combobox   

        childnamecombo.DataContext = ds.Tables["studentrecord"].DefaultView;

                // display values in combobox

        childnamecombo.SelectedValuePath = ds.Tables["studentrecord"].Columns["stid"].ToString();

                            }

            catch (Exception ex)

            {

                MessageBox.Show(ex.Message );

            }

            finally

            {

                sqlitead.Dispose();

                cmd.Dispose();

                SQLiteCon.Dispose();

            }