Error while saving the custom list created in visual studio 2010

Question

Dear all,
when I am saving the list; I am getting the following error. I do not know where this could be wrong.
Any inputs will help. Now I am recreating a new list - thinking that the objects used in the earlier projects might conflict. Also few lists in the list tab - were throwing error of the same type - which is not functioning well.
warm regards,
sathya

Answer 1

To give more information on the code
Registration list
//Reference this Feature's parent object, which is SPWeb in this case
SPWeb currentWeb = properties.Feature.Parent as SPWeb;
//Reference the lists we need to manipulate.
SPList tList = currentWeb.Lists["TrainingTrainers"];
SPList rList = currentWeb.Lists["TrainingRegistration"];
//Reference the fields in the lists
SPFieldCollection rFields = rList.Fields;
SPFieldCollection tFields = tList.Fields;
//changes to the Trainers List
SPField fullNameField = tList.Fields["Full Name"];
SPField emailAddressField = tList.Fields["E-mail Address"];
SPField firstNameField = tList.Fields["First Name"];
SPField lastNameField = tList.Fields["Last Name"];
fullNameField.Required = true;
emailAddressField.Required = true;
firstNameField.Required = true;
lastNameField.Required = true;
fullNameField.Update();
emailAddressField.Update();
firstNameField.Update();
lastNameField.Update();
//Changes to the Registrations List
//Change the Display Name of the Title Field
SPField titleField = rFields["Title"];
titleField.Title = "RegistrationID";
titleField.Update();
//Add additional fields as necessary
rFields.Add("First Name", SPFieldType.Text, true);
rFields.Add("Last Name", SPFieldType.Text, true);
rFields.Add("E-mail Address", SPFieldType.Text, true);
rFields.Add("Phone Number", SPFieldType.Text, false);
rFields.Add("ClassID", SPFieldType.Text, false);
rList.Update();
//Make the ClassID column read-only because i will set this later
SPField classIDField = rList.Fields["ClassID"];
classIDField.ReadOnlyField = true;
classIDField.Update();
//Remove the Attachments column from the default view of the Trainers list
SPView tDefaultView = tList.DefaultView;
tDefaultView.ViewFields.Delete("Attachments");
tDefaultView.Update();
//Add new fields to the default view of the Registrations list
SPView rDefaultView = rList.DefaultView;
rDefaultView.ViewFields.Delete("Attachments");
rDefaultView.ViewFields.Add("First Name");
rDefaultView.ViewFields.Add("Last Name");
rDefaultView.ViewFields.Add("E-mail Address");
rDefaultView.ViewFields.Add("Phone Number");
rDefaultView.Update();
Classes list
//Reference the newly created Classes list and perform the following
//1. Set the display name of the "Title" column to "ClassID" and hide it from the New and Edit Forms
//2. Make the "ClassID" column not required
//3. Set the default value of the Registrations column to 0 and do not display it on the new form
//4. Add the built-in "Start Date" and "End Date" columns
//Reference the newly created classes list
SPWeb currentWeb = properties.Feature.Parent as SPWeb;
SPList classesList = currentWeb.Lists["Classes-List"];
//Title column update
SPField titleField = classesList.Fields["Title"];
titleField.Required = false;
titleField.ShowInNewForm = false;
titleField.ShowInEditForm = false;
titleField.Title = "ClassID";
titleField.Update();
//Registrations column updates
SPField registrationField = classesList.Fields["Registrations"];
registrationField.DefaultValue = "0";
registrationField.ShowInNewForm = false;
registrationField.Update();
//Add the Start Date and End Date columns to the list, ensure they both display Date and Time, and add them to the default view of the list
SPFieldDateTime startDate = currentWeb.Fields["Start Date"] as SPFieldDateTime;
startDate.DisplayFormat = SPDateTimeFieldFormatType.DateTime;
SPFieldDateTime endDate = currentWeb.Fields["End Date"] as SPFieldDateTime;
endDate.DisplayFormat = SPDateTimeFieldFormatType.DateTime;
classesList.Fields.Add(startDate);
classesList.Fields.Add(endDate);
SPView defaultView = classesList.DefaultView;
defaultView.ViewFields.Add(startDate);
defaultView.ViewFields.Add(endDate);
defaultView.Update();
classesList.Update();
To my sense - when I add the classes list - the error pops up.
the classid is tied to both these lists.