Cannot read the row of the gridview - asp.net

Question

I am trying to read a gridview row. Here is my code:
protected void allStudents_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "Approval")
{
// *** Retreive the DataGridRow
int row = -1;
int.TryParse(e.CommandArgument as string, out row);
GridViewRow gdrow = allStudents.Rows[row];
// *** Get the underlying data item - in this case a DataRow
DataRow dr = ((DataTable)this.allStudents.DataSource).Rows[gdrow.DataItemIndex];
// *** Retrieve our context
string courseCode = dr["CourseCode"].ToString();
string courseNumber = dr["CourseNumber"].ToString();
string term = dr["Term"].ToString();
SqlConnection con = new SqlConnection();
con.ConnectionString = Userfunctions.GetConnectionString();
con.Open();
SqlCommand cmd = new SqlCommand("SELECT RegisterTable.StudentID, StudentTable.Name, StudentTable.Surname FROM RegisterTable JOIN StudentTable ON RegisterTable.StudentID = StudentTable.ID WHERE RegisterTable.CourseCode = #courseCode AND RegisterTable.Term = #term AND RegisterTable.CourseNumber = #courseNumber", con);
cmd.Parameters.AddWithValue("#courseCode", courseCode);
cmd.Parameters.AddWithValue("#courseNumber", courseNumber);
cmd.Parameters.AddWithValue("#term", term);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
transcript.DataSource = dt;
transcript.DataBind();
Label1.Visible = true;
Label1.Text = "Students who are registered to " + courseCode + " " + courseNumber + " are listed below:";
}
}
But at the line starting with
DataRow dr = ((DataTable)this.allStudents.DataSource).Rows[gdrow.DataItemIndex];
i get an error saying that "Object reference not set to an instance of an object.". Can anyone tell the problem here?
Note: allStudents is the ID of my gridview
Thanks

Answer 1

That error means of the references in the line is null. That is, you're trying to fetch data from an object, but there is no instantiated object there.
Recommendation: insert a breakpoint at that line. Debug the code. Then, while debugging and on that line, pass the mouse cursor over every name in the line. The debugger will give you rich, detailed information about each and every variable. Once you find which one is null (could be either allStudents or DataSource), you'll be on the right track to solve your problem.
You have to find out whatever is triggering the error for being null, then making sure it's not null when you get to that point in your code.