The simplest way to recreated the error was to preview the form but you would get the same error if you published and ran the workflow.
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
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.
[NullReferenceException: Object reference not set to an instance of an object.]
Nintex.Forms.SharePoint.ApplicationPages.PreviewNintexForm.OnInit(EventArgs e) +645
System.Web.UI.Control.InitRecursive(Control namingContainer) +186
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2098
After "debugging" the Nintex's code I determined that it was using a XML file stored in a hidden SharePoint list. This XML file contained the form as well as the workflow variable definitions.
It turns out that earlier versions of Nintex stored blank variables as
All I had to do was go through all the workflow variables and assign them a single space as the default value and the forms started working again.