fix: new json structure parsing bug

app/lib/data_classes.dart

@@ -69,8 +69,8 @@ class SubDiv {
       return SubDiv(
         sub_div_id: json['SubdivID'].toString(),
         groupID: json['GroupID'] ?? 0,
-        minThickness: json['MinThickness'].toDouble ?? 0,
-        avgThickness: json['AvgThickness'].toDouble ?? 0,
+        minThickness: (json['MinThickness'] as num?)?.toDouble() ?? 0,
+        avgThickness: (json['AvgThickness'] as num?)?.toDouble() ?? 0,
         center: json['CenLatitude'] != null && json['CenLongitude'] != null
             ? LatLng(json['CenLatitude'], json['CenLongitude'])
             : LatLng(0.0, 0.0),