I'm betting you took old customization code that was
- Compiled in an older release,
- Not re-compiled with the latest release,
- Using an unsupported API (the sessionContext field is not supported), and
- Tried to run it in 10.2 M010
If so, then that explains everything there is no such publicly accessible field in 10.2 M010. There is, however, a getSessionContext() method -- though it also is not a supported API currently.
[Unsupported APIs are subject to change at any time without notice.]