I realize this is will be very vague, but there are times when I go to make a feature the current feature by calling the "Feature.MakeCurrentFeature()".
And it would give me a memory access violation.
Try doing it intercactively & see if you get a better error message.
If this is all buried in code, "catch" the error, output a message saying which feature you are using, and exit your API code, then try to do it interactively.
Have you looked into the syslog (menu location depends on NX version, but it is something like "Help" -> "NX log file"
Production: NX10.0.3.5 MP5 + patch/TC11.2
I'd rather be e-steemed than e-diseaseled