Hi Lindsayx,
I encountered the same problem when a user tried to update the Next Gen Pro plugin from within the Dashboard automatically. Doing so throws the error which will whitescreen of death the site.
After investigating, the issue was that the standard Next Gen Gallery install was version 2.0.40 at the time (not the latest). If you install the latest Pro plugin with this older version of Next Gen Gallery then you get the error. Updating the standard Next Gen Gallery plugin to the latest version (2.0.66+) and then updating the Pro plugin results in a successful update and you don’t see the error.