Hi,

Please try the solution for this issue that is discussed on this thread.
 

The solution solved my problem indirectly. 

#1it lead me to Nintex Forms All Documents Page which was essential

#2 I did not want to select publish because I was not sure if it was going to overwrite my current                published form.

#3 So I deleted all the minor versions, and I was able to use the preview function again.

This solution was a lifesaver Thank You!!!!!