Avoid uninitialized provider type

app/src/main/java/org/traccar/client/PositionProvider.java

@@ -57,7 +57,7 @@ public abstract class PositionProvider {
         deviceId = preferences.getString(MainActivity.KEY_DEVICE, null);
         period = Integer.parseInt(preferences.getString(MainActivity.KEY_INTERVAL, null)) * 1000;
 
-        type = preferences.getString(MainActivity.KEY_PROVIDER, null);
+        type = preferences.getString(MainActivity.KEY_PROVIDER, "gps");
     }
 
     public abstract void startUpdates();

app/src/main/java/org/traccar/client/TrackingController.java

@@ -64,7 +64,7 @@ public class TrackingController implements PositionProvider.PositionListener, Ne
         this.context = context;
         handler = new Handler();
         preferences = PreferenceManager.getDefaultSharedPreferences(context);
-        if (preferences.getString(MainActivity.KEY_PROVIDER, null).equals("mixed")) {
+        if (preferences.getString(MainActivity.KEY_PROVIDER, "gps").equals("mixed")) {
             positionProvider = new MixedPositionProvider(context, this);
         } else {
             positionProvider = new SimplePositionProvider(context, this);