Error: Unable to get property of undefined or null reference

I get error when i try to run this code below

Unhandled exception at line 6, column 11119 in http://localhost:2300/Scripts/WebForms/MsAjax/MicrosoftAjaxWebForms.js

0x800a138f - JavaScript runtime error: Unable to get property 'PRM_TimeoutError' of undefined or null reference

my Code

If DropDownList1Class.SelectedItem.Text = "P1" And DropDownList3Subject.SelectedValue = "English" Then
    Try
        Dim row As GridViewRow = TryCast((TryCast(sender, TextBox)).NamingContainer, GridViewRow)
        Dim txtscore As String = (TryCast(row.FindControl("score"), TextBox)).Text
        '  Dim txtSourceAddress As String = (TryCast(e.Row.FindControl("txtsourceaddress"), TextBox)).Text
        If String.IsNullOrEmpty(txtscore) Then
            Dim txtAggr As TextBox = TryCast(row.FindControl("Agg"), TextBox)
            txtAggr.Text = ""
            Dim txtRemarks As TextBox = TryCast(row.FindControl("Remarks"), TextBox)
            txtRemarks.Text = ""
            Dim kk As String = row.RowIndex
            DirectCast(StudentGridMarks.Rows(kk).Cells(1).FindControl("checkbox1"), CheckBox).Checked = True
            hfRowIndex.Value = row.RowIndex.ToString()
            hfCellIndex.Value = "5"
        Else
 
            Dim txtAgg As TextBox = TryCast(row.FindControl("Agg"), TextBox)
            Dim txtRemarks As TextBox = TryCast(row.FindControl("Remarks"), TextBox)
            txtAgg.Text = gradingE(Convert.ToDouble(TryCast(sender, TextBox).Text))
            txtRemarks.Text = hmcomment(TryCast(sender, TextBox).Text)
            Dim datetimer As TextBox = TryCast(row.FindControl("DateTime"), TextBox)
            datetimer.Text = Date.Now
            Dim txtusername As TextBox = TryCast(row.FindControl("username"), TextBox)
            txtusername.Text = CurrentLogin.Text
            ' Dim txtAggrt As TextBox = TryCast(row.FindControl("score"), TextBox)
            ' txtAggrt.Focus()
            Dim kk As String = row.RowIndex
            DirectCast(StudentGridMarks.Rows(kk).Cells(1).FindControl("checkbox1"), CheckBox).Checked = True
            'Dim rowt As GridViewRow = TryCast(TryCast(sender, TextBox).NamingContainer, GridViewRow)
            hfRowIndex.Value = row.RowIndex.ToString()
            hfCellIndex.Value = "5"
        End If
    Catch ex As Exception
        Console.WriteLine("{0} Exception caught.", ex)
    End Try

When i reach here the error flags

Dim kk As String = row.RowIndex
DirectCast(StudentGridMarks.Rows(kk).Cells(1).FindControl("checkbox1"), CheckBox).Checked = True