Just thought I'd share a little issue I've had which has been worked around with the help of Nintex support.
I created a SharePoint 2013 site workflow using a Nintex start form.
The form had a list lookup control that showed a list of activities groups from a SharePoint list.
The control had the ID connected to bound to the variable 'txt-GroupID' and the text connected to mapped to the variable 'txt-Group'.
When a user completes the start form (via the nintex start form web part on a site page),
it would show the error.
"Object reference not set to an instance of an object."
If the control was not bound to the variables the form would submit correctly.
The work around was to remove the hyphens from my variable names.
I've been a long fan of a variable naming convention that includes the variable type. (as the example about). In this case the hyphen I use in the naming convention has caused the problem.
Thanks to @SimonMuntz for providing the solution.
Hope that this helps someone else out there when they see this error message.