During the course of opening, editing, or attempting to save a part, you may experience the error: "Object reference not set to an instance of an object."
This error message can come from several issues, but the most common cause is that there was geometry created using a circular dependency of features, then something was removed from the dependency and is no longer there. This can happen when something in the design has been suppressed, deleted, or edited out of a feature. For this reason, a designer should be very cautious of removing features which have dependencies during their workflow.
It's hard to correct this issue once it occurs. You should most likely revert back to your most recent backup and rebuild the latest portion of the design.