MainApplication.java

import com.dieam.reactnativepushnotification.ReactNativePushNotificationPackage;  // <--- THIS

public class MainApplication extends Application implements ReactApplication {

  private final ReactNativeHost mReactNativeHost = new ReactNativeHost(this) {
      @Override
      protected boolean getUseDeveloperSupport() {
        return BuildConfig.DEBUG;
      }

      @Override
      protected List<ReactPackage> getPackages() {

      return Arrays.<ReactPackage>asList(
          new MainReactPackage(),
          new ReactNativePushNotificationPackage() // <---- & THIS      
		  );
    }
  };

  ....
}