I recently ran into a nebulous problem when creating a windows app with vs.net 2005.
I would select the DataBindings or DataSource properties from the proeprties window (click the dropdown) and VS would give me an “Object reference not set to an instance of an object” error.
I finally figured out the problem was caused by a recent rename of a misspelled class.
This class was being used by one of the datasources. As soon as I changed the name of that class, the datasource became invalid, and I guess when VS tries to get a list of the these datasources for the dropdown it just blows up.
Once I deleted the problematic datasource everything went back to normal.