Items collection cannot be modified when the DataSource prop

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 this problem if possible?

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