I have created a custom field tyep which is the same as the standard 'textfield' just with a different name. I have done this to target specific text fields with css.

The form fails to save when the custom FieldType is implented.

public class MonetaryTextField : FieldType
  {
    public List<object> Value;
    public MonetaryTextField()
    {
      Id = new Guid("57ADD3AB-C3CD-464D-BCC2-3D1C05D3BF65");
      Name = "Monetary Text Field";
      Description = "Adds monetary formating to the form field";
      Icon = "textfield.png";
      DataType = FieldDataType.String;
      Value = new List<object>();
    }
    public override string RenderPreview()
    {
      return "<input type=\"text\" />";
    }
    public override string RenderPreviewWithPrevalues(List<object> prevalues)
    {
      return RenderPreview();
    }
    public override WebControl Editor
    {
      get
      {
        System.Web.UI.WebControls.TextBox tb = new System.Web.UI.WebControls.TextBox();
        tb.TextMode = System.Web.UI.WebControls.TextBoxMode.SingleLine;
        tb.CssClass = "monetary-field";
        return tb;
      }
      set
      {
        base.Editor = value;
      }
    }
  }

On submit I am getting:

Object reference not set to an instance of an object.

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

   Umbraco.Forms.Data.Storage.RecordFieldValueStorage.InsertRecordFieldValues(RecordField rf) +677

   Umbraco.Forms.Data.Storage.RecordFieldStorage.InsertRecordField(RecordField recordfield) +1196

   Umbraco.Forms.Data.Storage.RecordStorage.InsertRecord(Record record, Form form) +1865

   Umbraco.Forms.Core.Services.RecordService.() +205

   Umbraco.Forms.Core.Services.RecordService.Submit() +39

   Umbraco.Forms.Core.Services.RecordService.NextPage() +169

   Umbraco.Forms.UI.Usercontrols.RenderForm.nextPage(Object sender, EventArgs e) +241

   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +153

   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3706

What am I doing wrong? Any help would be appreciated.