Maestro: expression editor throws MgNullPropertyValueException

[jbirch]

When trying to enumerate values in a particular property in the expression editor, an MgNullPropertyValueException is thrown if any null values exist for that property.

This makes it difficult to work with that that contains these values; the current workaround I'm using is FDO Toolbox, but this can be difficult when the data isn't local.

It would be great if Maestro's implementation of the expression editor could be modified to suppress these exceptions or, preferably, insert NULL as one of the choices in the value picker.