That's it :
System.NullReferenceException: Object reference not set to an instance of an object.
Not useful, only the Exception matters. Here, System.NullReferenceException.
Anyway, this crashlog doesn't mean anything, it crashed because you didn't had enough memory to create something in memory, and some part of the code called it, as it called something that didn't existed, lo and behold, it crashed.
About your memory error, having acquired it in Paint.NET doesn't mean one has enough memory to save it, or to open it later, because you need to have one big free chunk in memory to hold it. One may have more than one free gigabyte of memory, but divided in several small pieces.
Finally, scanning something at 1200dpi generally output a big image, what is the side of your picture, in pixels?