Bugfix: NPE in GPXTrackExporter.

Introduced in 0f489f1cc9

Fixes #1176.

src/main/java/de/dennisguse/opentracks/data/models/Distance.java

@@ -1,6 +1,7 @@
 package de.dennisguse.opentracks.data.models;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import java.util.Objects;
 
@@ -14,6 +15,14 @@ public class Distance {
         return of(Float.parseFloat(distance_m));
     }
 
+    @Nullable
+    public static Distance ofOrNull(Double distance_m) {
+        if (distance_m == null) {
+            return null;
+        }
+        return of(distance_m);
+    }
+
     public static Distance ofMile(double distance_mile) {
         return of(distance_mile * UnitConversions.MI_TO_M);
     }

src/main/java/de/dennisguse/opentracks/io/file/exporter/GPXTrackExporter.java

@@ -34,10 +34,10 @@ import java.util.stream.Stream;
 
 import de.dennisguse.opentracks.data.ContentProviderUtils;
 import de.dennisguse.opentracks.data.TrackPointIterator;
+import de.dennisguse.opentracks.data.models.Distance;
 import de.dennisguse.opentracks.data.models.Marker;
 import de.dennisguse.opentracks.data.models.Track;
 import de.dennisguse.opentracks.data.models.TrackPoint;
-import de.dennisguse.opentracks.data.models.Distance;
 import de.dennisguse.opentracks.stats.TrackStatistics;
 import de.dennisguse.opentracks.util.StringUtils;
 
@@ -358,7 +358,7 @@ public class GPXTrackExporter implements TrackExporter {
                     trackPointExtensionContent += ("<opentracks:loss>" + ALTITUDE_FORMAT.format(cumulativeLoss) + "</opentracks:loss>\n");
                 }
 
-                cumulativeDistance = Distance.of(cumulateSensorData(trackPoint, sensorPoints, (tp) -> tp.hasSensorDistance() ? tp.getSensorDistance().toM() : null));
+                cumulativeDistance = Distance.ofOrNull(cumulateSensorData(trackPoint, sensorPoints, (tp) -> tp.hasSensorDistance() ? tp.getSensorDistance().toM() : null));
                 if (cumulativeDistance != null) {
                     trackPointExtensionContent += ("<opentracks:distance>" + DISTANCE_FORMAT.format(cumulativeDistance.toM()) + "</opentracks:distance>\n");
                     trackPointExtensionContent += ("<cluetrust:distance>" + DISTANCE_FORMAT.format(trackDistance.plus(cumulativeDistance).toM()) + "</cluetrust:distance>\n");