RoboGuice injection

Question

I am using this code in my Android app (RoboGuice library):

@Singleton
public class UserSessionProvider {

   private User user;

   @Inject
   private Injector injector;

   public synchronized User createSession() {
      this.user = injector.getInstance(User.class);
      return this.user;
   }

   public synchronized User getUser() {
      return user;
   }

   public synchronized void destroySession() {
      this.user = null;
   }

   public synchronized Boolean isSessionAlive() {
      return getUser() != null;
   }
}

Sometimes build gets bugged up, and application fails to inject instance of injector in this provider. When I clean app and rebuild it then it again works ok (for some time). I am using Gradle build chain and Android studio. Does anyone know how can I stop this from happening?