Avoid registering the sensor if it is null (#12)

Author: rocboronat

library/src/main/java/com/schibsted/spain/parallaxlayerlayout/SensorTranslationUpdater.java

@@ -44,8 +44,10 @@ public void unSubscribe() {
 
   public void registerSensorManager() {
     if (sensorManager != null) {
-      sensorManager.registerListener(this,
-          sensorManager.getDefaultSensor(Sensor.TYPE_ROTATION_VECTOR), DEFAULT_SAMPLING_PERIOD);
+      Sensor rotationVectorSensor = sensorManager.getDefaultSensor(Sensor.TYPE_ROTATION_VECTOR);
+      if (rotationVectorSensor != null) {
+        sensorManager.registerListener(this, rotationVectorSensor, DEFAULT_SAMPLING_PERIOD);
+      }
     }
   }
 
@@ -192,4 +194,4 @@ public void setTiltSensitivity(float tiltSensitivity) {
 
     mTiltSensitivity = tiltSensitivity;
   }
-}
+}