[Solved] System.NullReferenceException: Object reference not set to an instance of an object

Error: Object reference not set to an instance of an object.

Server Error in '/' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 169:  { 
Line 170:
Line 171:      String strConnString = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
Line 172:      SqlConnection conn4 = new SqlConnection(strConnString);
Line 173:      SqlCommand cmdD = new SqlCommand();

if ((balance != 0) && (amountpaid != 0) && (amountpaid < balance))
{ 
    String strConnString = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
    SqlConnection conn4 = new SqlConnection(strConnString);
    SqlCommand cmdD = new SqlCommand();
    cmdD.CommandType = CommandType.Text;
    cmdD.CommandText = "UPDATE Payment SET Payment_Status=@Payment_Status, Payment_Method=@Payment_Method, Bank_Name=@Bank_Name,Account_Number=@Account_Number, @AmountPaid=@AmountPaid, Balance=@Balance WHERE Receipt= @Receipt";
    cmdD.Parameters.AddWithValue("@Receipt", txtrecipt2.Text.Trim());
    cmdD.Parameters.AddWithValue("@Payment_Status", ddlstatus.SelectedItem.Text.Trim());
    cmdD.Parameters.AddWithValue("@Payment_Method", ddlmethode.SelectedItem.Text.Trim());
    cmdD.Parameters.AddWithValue("@Bank_Name", txtbank.Text.Trim());
    cmdD.Parameters.AddWithValue("@Account_Number", txtaccountno.Text.Trim());
    cmdD.Parameters.AddWithValue("@AmountPaid", txtamountpaid.Text.Trim());
    cmdD.Parameters.AddWithValue("@Balance", totam);
    cmdD.Connection = conn4;
    conn4.Open();
    inserted = cmdD.ExecuteNonQuery();
    // txtphone.Text = sdr["Phone"].ToString();
    conn4.Close();
    //BindGridreport();
}
else
{
    // ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);
}
ScriptManager.RegisterClientScriptBlock(btnsave, this.GetType(), "alert", "<script>alert('Data Submitted Successfully ... !!')</script>", false);