Null Arguments are invalid error

Hi folks, 

 

I am getting the following pop-up error while submitting a form. I have checked that all the values in null checks (also the one on the submit button) contain values and are not null. The interface does not show any errors on the designer as well or on the process model. 

Any ideas on what might be causing this? 

 

 

Any help would be appreciated. TIA :)

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