Found in DEV300_m38. (Regression)

In the sorting dialog (Data|Sort...), after having chosen some sort criteria,
you cannot reset the sort criteria to "-undefined-" because the item
"-undefined-" is no longer part of the drop down list.

For example in the following case this is very problematic:

A user calls the sorting dialog several times in order to do different sortings
for different aspects. The first time, when the user called the sorting dialog,
he may have chosen three sort criteria. The second time, when the user calls the
sorting dialog the sort criteria are preset according to the action before. But
this time the user wants to do some sorting, that´s defined by only one other
sort criteria. However he is not able to get rid of the presets for the second
and third sort criteria.

Also see the attached screenshots.