Hi, thanks for posting!
There were no deliberate changes in v2.14 to make this throw an exception on null being written using Debug.Write, though looking at the code I think it may be possible that this is a regression in v2.14. I've noted it down to be fixed in the next release.
I recommend adjusting your code to avoid the null writes for the time being. Thanks for making me aware of this.
Cheers,
Remco