Dependency injection with Play Framework and Guice leads to NullPointerException on Application start

I have following code:

import javax.inject.Inject;
import models.Subject;
import models.dao.SchoolDao;
import models.dao.SubjectDao;
import play.Application;
import play.GlobalSettings;
import play.db.jpa.JPA;
import play.db.jpa.Transactional;

import com.google.inject.AbstractModule;
import com.google.inject.Guice;
import com.google.inject.Injector;


public class Global extends GlobalSettings{

private Injector injector;

@Inject
private SubjectDao subjectDao;

@Override
public void onStart(Application application) {
    injector = Guice.createInjector(new AbstractModule() {
        @Override
        protected void configure() {
            bind(SchoolDao.class);
            bind(SubjectDao.class);
        }
    });

    insertInitialData();
}

@Override
public <A> A getControllerInstance(Class<A> aClass) throws Exception {
    return injector.getInstance(aClass);
}

@Transactional
private void insertInitialData(){

    if(subjectDao.countAll() == 0){
        Subject deutsch = new Subject();
        deutsch.setName("Deutsch");
        subjectDao.create(deutsch); 

        Subject englisch = new Subject();
        deutsch.setName("Englisch");
        subjectDao.create(englisch);

        Subject mathe = new Subject();
        deutsch.setName("Mathematik");
        subjectDao.create(mathe);

        Subject hab = new Subject();
        deutsch.setName("Hausaufgabenbetreuung");
        subjectDao.create(hab);
    }
}

}

As you see I want to seed my database with initial values on startup. I have a data access object "subjectDao" which should be injected. This works on other positions of my application but due to some reason not on applicatin startup. I get a NullPointerException and my subjectDao is null. Has someone an idea.