Items collection cannot be modified when the DataSource property is set Error

I have two windows form

error show when i need to pass data from windows form2 in textbox
to windows form1 combobox
First windows Form
found combobox inside windows form1 and I show data in it in load event of form

public DataTable ShowExceltwoLanguage()
{

            OleDbConnection con = new OleDbConnection(connection);


            con.Open();
            string str = "select MemberID,MemberNameAR + ' - ' + MemberNameEN as MemberName from [MemberAR$]";
            OleDbCommand com = new OleDbCommand();
            com = new OleDbCommand(str, con);
            OleDbDataAdapter oledbda = new OleDbDataAdapter();
            oledbda = new OleDbDataAdapter(com);
            DataSet ds = new DataSet();
            ds = new DataSet();
            oledbda.Fill(ds, "[MemberAR$]");
            con.Close();
            DataTable dt = new DataTable();
            dt = ds.Tables["[MemberAR$]"];
            return dt;


}

in load event of windows form1 i write as following

 QrClasses qrc = new QrClasses();
  DataTable dt = qrc.ShowExceltwoLanguage();
  comboBox4.DataSource = dt;
  comboBox4.DisplayMember = "MemberName";
  comboBox4.ValueMember = "MemberID";

Second windows form
here is error
Items collection cannot be modified when the DataSource property is set show
in button1 of form2 click event

 if (!string.IsNullOrWhiteSpace(textBox1.Text))
                {

                       
                        var cb = ((Form1)Owner).comboBox4;
                        var index = cb.FindString(textBox1.Text);
                        if (index == -1)
                        {
                            cb.Items.Add(textBox1.Text);// in this line Additional information: Items collection cannot be modified when the DataSource property is set.
                            index = cb.FindString(textBox1.Text);
                            if (index > -1)
                            {
                                cb.SelectedIndex = index;
                                Close();
                            }
                        }
                        else
                        {

                            cb.SelectedIndex = index;
                        }
                    }

so that how to solve that if possible ?