Null Reference when submitting Umbraco Forms to custom data source

I'm trying to make Umbraco Forms (Umbraco v7.5.3) save to my own data source. I followed this guide: http://www.nibble.be/?p=84

And it appears to work-ish. I can connect and generate a form based on the table. Which is nice.

My setup

1. I add a new forms data source with the connection string:

   Provider=SQLNCLI11;Server=(localdb)\MSSQLLocalDB;Database=test;Integrated Security=SSPI;
   

2. My table looks like this:

   The Id (PK) property is configured as Identity

3. I choose some fields I want to use in the form:

4. I choose which data types it should use:

   I'm a bit puzzled why the Id field is there. I didn't select it, and the guide I followed didn't have this field either :/?

5. It generates this form for me:

Error

But when I try to submit form data I get the following error. Which isn't nice.

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:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
Umbraco.Forms.Core.Providers.DatasourceTypes.MsSql.InsertRecord(Record record) +1750
Umbraco.Forms.Web.Services.RecordService.storeRecord(Record record, Form form) +1574
Umbraco.Forms.Web.Services.RecordService.Approve(Record record, Form form) +343 Umbraco.Forms.Web.Services.RecordService.Submit(Record record, Form form) +565
Umbraco.Forms.Web.Controllers.UmbracoFormsController.SubmitForm(Form form, FormViewModel model, Dictionary2 state, ControllerContext context) +2245
Umbraco.Forms.Web.Controllers.UmbracoFormsController.GoForward(Form form, FormViewModel model, Dictionary2 state) +205
Umbraco.Forms.Web.Controllers.UmbracoFormsController.HandleForm(FormViewModel model, Boolean captchaIsValid) +785 lambdamethod(Closure , ControllerBase , Object[] ) +143
System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +14
System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary2 parameters) +157
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary2 parameters) +27
System.Web.Mvc.Async.AsyncControllerActionInvoker.b39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +22
System.Web.Mvc.Async.WrappedAsyncResult2.CallEndDelegate(IAsyncResult asyncResult) +29
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32
System.Web.Mvc.Async.AsyncInvocationWithFilters.b3d() +50 System.Web.Mvc.Async.<>cDisplayClass46.b3f() +225 System.Web.Mvc.Async.<>cDisplayClass46.b3f() +225 System.Web.Mvc.Async.<>cDisplayClass46.b3f() +225 System.Web.Mvc.Async.<>cDisplayClass46.b3f() +225 System.Web.Mvc.Async.<>cDisplayClass46.b3f() +225 System.Web.Mvc.Async.<>cDisplayClass33.b32(IAsyncResult asyncResult) +10
System.Web.Mvc.Async.WrappedAsyncResult1.CallEndDelegate(IAsyncResult asyncResult) +10
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34
System.Web.Mvc.Async.<>cDisplayClass2b.b1c() +26 System.Web.Mvc.Async.<>cDisplayClass21.b1e(IAsyncResult asyncResult) +100
System.Web.Mvc.Async.WrappedAsyncResult1.CallEndDelegate(IAsyncResult asyncResult) +10
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27
System.Web.Mvc.Controller.b1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +13
System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +29
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +36 System.Web.Mvc.Controller.b15(IAsyncResult asyncResult, Controller controller) +12
System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +22
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +26
System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
System.Web.Mvc.MvcHandler.b_5(IAsyncResult asyncResult, ProcessRequestState innerState) +21
System.Web.Mvc.Async.WrappedAsyncVoid1.CallEndDelegate(IAsyncResult asyncResult) +29
System.Web.Mvc.Async.WrappedAsyncResultBase1.End() +49
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +28 System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9765121 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

Questions

• Why is the Id field added to my form when I didn't select it?
• How is the form linked to my table? When I create a new form I cannot set a data source for it, and I cannot find any config on the generated form that points to my data source.
  • How are form fields linked to a table column? Is it via the "display name"?
• Why do I get the above Null Reference when trying to submit my form?