Hi,
Since we don't have a readily available Unity sample app for Native Advanced Ads, I created
this modified sample app just for the purpose of debugging your issue. This sample app just contains a "Request Native Ad" button that will send an Ad request for Native App Install Ad and will just display the headline text. In this sample app, I was able to register the GameObject for headline ad asset successfully and log the boolean result of registering the GameObject for headline ad asset without encountering any crashes or exceptions.
Based on the error log you've captured, particularly the NullReferenceException, you seem to be accessing a method or property of a reference that is null. Given that you were able to load the native ad, get the native ad assets and log the result of registering the ad assets, then there might be something else in your code that is triggering the exception.
For us to further check on this, could you provide to us a miniature sample app where you can reproduce the exception encountered? You may share this to us privately using the Reply privately to author option?
Regards,
Ivan Bautista
Mobile Ads SDK Team