I was recently asked to investigate an issue with Sitecore’s Web Forms For Marketers, where a null object exception would be thrown when a form was submitted. Investigating the logs I found the following line:
Sitecore.Forms.Mvc.Controllers.ModelBinders.FormModelBinder.BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
As it turns out, Sitecore MVP Kamruz Jaman has already solved the problem, and you can see the answer here: http://stackoverflow.com/questions/26094198/save-action-on-webforms-for-marketers-throws-exeption
Essentially what you need to do is point your MVC Form rendering’s data source at the form you need to render and not the FormID field in the rendering parameters.
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I hope this helps some people save a few minutes’ investigation 🙂