How to get distance from center

[tchouaffe]

I would like to know how to get the distance from the center.
When I do the below I get A value of type 'Object?' can't be assigned to a variable of type 'Map<String, dynamic>'. on the first line. document is of type DocumentSnapshot.

 Map<String, dynamic> snapData = document.data();
 final GeoPoint point = snapData['position']['geopoint'];