Not able to add the fragments using Kotlin Extension function

Android
1

I have a Kotlin extension function to add a fragment which is in another Kotlin file

fun Fragment.addFragment(tag: String?, id: Int, fragmentManager: FragmentManager) {
  // will take care of adding the fragment.
  val fragmentTransaction = fragmentManager.beginTransaction()
  fragmentTransaction.add(id, this, tag)
  fragmentTransaction.addToBackStack(tag)
  fragmentTransaction.commit()
}

But whenever I am using that extension function to add a fragment then I am getting this crash

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.Fragment.setNextAnim(int)' on a null object reference

I am using that extension function like this:

MyFragment().addFragment("MyFragment", R.id.frame, fragmentMaganer)

After the above statement if I tried to debug my code then its executed successfully till

fragmentTransaction.commit()

but after that in the onStart() method of my activity I am facing that NullPointerException issue

But if I use the same code directly in Activity like this:

val myFragment = MyFragment()
val fragmentTransaction = fragmentManager.beginTransaction()
fragmentTransaction.add(R.id.frame, myFragment, tag)
fragmentTransaction.addToBackStack(tag)
fragmentTransaction.commit()

then Its working fine. Can someone please explain why its no working with Extension function?