I'm not Aspose support, but the code you wrote should work. Please check the path to your file (the doc file). It seems to me that your doc variable is null at the time when you call Save().
I use code like yours often and it works fine. :-)
It is definitely something with the way Aspose is handling the .docx file - I know it’s processing the file because it spit out two png’s, but then failed somewhere along the way. (the .html file is zero bytes)
Thanks for reporting this issue to us. I tested the scenario and have managed to reproduce the same exception on my side. For the sake of correction, I have logged this problem in our issue tracking system. The ID of your issue is WORDSNET-9938. We will further look into the details of this problem and keep you updated on the status of correction. We apologize for your inconvenience.