I think it has to do with ContentPicker..but can't figure this out.

    [DocumentTypeProperty(
        UmbracoPropertyType.ContentPicker,
        Name = "Reseller Page",
        Tab = TabNames.Settings)]
    public int ResellerPage { get; set; }

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.] Vega.USiteBuilder.ManagerBase.SynchronizeContentTypeProperties(Type typeContentType, IContentType contentType, DocumentTypeAttribute documentTypeAttribute, Boolean& hadDefaultValues, Boolean updateMixins) +756 Vega.USiteBuilder.DocumentTypeManager.SynchronizeDocumentType(Type typeDocType, Type baseTypeDocType) +934 Vega.USiteBuilder.DocumentTypeManager.SynchronizeDocumentTypes(Type baseTypeDocType) +111 Vega.USiteBuilder.DocumentTypeManager.Synchronize() +464 Vega.USiteBuilder.UmbracoManager.Synchronize() +159 Vega.USiteBuilder.UmbracoManager.SynchronizeIfNotSynchronized() +121 System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +135 System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +194 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +87

It seems that UmbracoPropertyType.ContentPicker id its different from the DataType in BackOffice...but how did this happend?