Problem with dataGridView1_CellEnter

Hello
After using

DataGridViewRichTextBox.DataGridViewRichTextBoxColumn x1 = new DataGridViewRichTextBox.DataGridViewRichTextBoxColumn();
dataGridView1.Columns.Insert(0, x1);

dataGridView1_CellEnter has this error:
 
System.NullReference Exception: object reference not set to an instance of an object at windows Forms From Application1.form1.form1_load( );