String reference not set to an instance of a string. Parameter name: s C#

Question

I am trying to update the data using the picturebox button in the data gridview. Here is my code. Here is where the error is.

private void dgv_Member_CellContentClick(object sender, DataGridViewCellEventArgs e)
{   
    string colName = dgv_Member.Columns[e.ColumnIndex].Name;
    if (colName == "colEdit")
    {
        AddNewMember add = new AddNewMember(this);
        add.btnSave.Enabled = false;
        add.btnUpdate.Enabled = true;
        add.txtAcctno.Enabled = false;
        add.txtAcctno.Text = _AccountNo;
        add.dtpBDate.Value = DateTime.Parse(_Birthdate);
        add.dtpMembership.Value = DateTime.Parse(_DateofMembership);
        add.ShowDialog();

But I got this error:

String reference not set to an instance of a string. Parameter name: s

The error pointed at:

add.dtpBDate.Value = DateTime.Parse(_Birthdate);
add.dtpMembership.Value = DateTime.Parse(_DateofMembership);