datagridview combobox c# winforms

Question

sir,

      my name is Babin raj i am a btec student ,i am also working as a freelancer i have a doubt on datagridview combobox .the problem is that when i trys to set a datasource to  combobox cell dynamically the color of the combobox  dropdown list changes to black color     below is the code i have used

private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)

{

         int n = dataGridView1.CurrentCell.RowIndex;

         if (dataGridView1.CurrentCell.ColumnIndex == 0)

         {

               var cbCell = dataGridView1.Rows[n].Cells["category"] as DataGridViewComboBoxCell;  

              DataTable dt = c1.ret("select category from category").Tables[0];

               cbCell.DataSource = dt;

               cbCell.ValueMember = "category";

               cbCell.DisplayMember = "category";

               cbCell.FlatStyle = FlatStyle.System;

         }

}

 please help me i have no other source to get the answer for my question i have tryed couple of  code from  internet but the problem is not fixed