Avoid NullPointer in creating photoUrl when importing

src/main/java/de/dennisguse/opentracks/io/file/importer/AbstractFileTrackImporter.java

@@ -378,7 +378,7 @@ abstract class AbstractFileTrackImporter extends DefaultHandler implements Track
 
         Uri photoUri = FileUtils.getUriForFile(context, file);
 
-        return photoUri.toString();
+        return "" + photoUri;
     }
 
     /**

src/main/java/de/dennisguse/opentracks/io/file/importer/KmlFileTrackImporter.java

@@ -186,7 +186,7 @@ public class KmlFileTrackImporter extends AbstractFileTrackImporter {
             return;
         }
 
-        // If there is photoUrl it has to be changed because that url in kml file isn't a valid content:// to the imported one.
+        // If there is photoUrl it has to be changed because that url in kml file is a relative path to the internal kmz file.
         if (photoUrl != null) {
             photoUrl = getPhotoUrl(Uri.parse(photoUrl).getLastPathSegment());
         }