c# - Check if datagridview cell is null or empty

I have to change the background color of the cells , when their value is string or empty, this is the code i have write similar to other codes here :

for (int rowIndex = 0; rowIndex < dataGridView1.RowCount; rowIndex++)
            {
             string conte = dataGridView1.Rows[rowIndex].Cells[7].Value.ToString()  ;
                if (string.IsNullOrEmpty(conte))
                {
                // dataGridView1.Rows[rowIndex].Cells[7].Style.BackColor = Color.Orange;
                }
                else
                 { dataGridView1.Rows[rowIndex].Cells[7].Style.BackColor = Color.Orange; }

        } 

the dataset is complete , show me the datagridview populated and the show me this error:

How can i fix this?? Have another way to write the code?