This property is null when there are no layers with an active range in the map. When the Range property is not null, but both the Min and Max properties of the RangeExtent are null this means the map view is showing all values. This is equivalent to disabling range in the map view. Setting this property to null when there are layers with an active range in the map will set both the min and max properties of the RangeExtent to null.

Reference

MapView Class
MapView Members