Few things:

1) No need to hold reference for supportFragmentManager, use it directly (because i am not sure if it will be null when Activity is initialized)

2) Try removing addToBackStack(null) and using findFragmentByTag("DEVICE_FRAGMENT")

3) Most importantly, Don’t try to access “things” of Fragment from Activity, do those Adapter initialization/fill in the Fragment itself. Because Fragment has its own lifecycle and you may try to access “things” at wrong lifecycle

i am currently programming an app in Android Studio and i am having a big issue. The main problem is, that i want an activity with a fragment in it and this fragment has got a spinner. I wanted to find the spinner by id, but it always returned null and i read that i can’t use findViewById if it is not in the ContentView i just set. So i am currently trying to find the fragment that contains the spinner, but i also can’t find the fragment, i tried findFragmentById and findViewById from the FragmentManager. I always get a TypeCastException and if i try findFragmentById(…)!! it throws a NullPointer.

This is my MainActivity:

    class MainActivity : AppCompatActivity() {

        private val manager: FragmentManager? = supportFragmentManager

        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_main)
            showDeviceFragment()

            val fragment = manager!!.findFragmentById(R.id.fragment_holder) as DeviceFragment
            val options = arrayOf("Wandhalterung (Arm)", "Gestellhalterung (Arm)", "Gestellhalterung")
            fragment.option.adapter = ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, options)
    }

    fun showDeviceFragment() {
        val transaction = manager!!.beginTransaction()
        val fragment = DeviceFragment()
        transaction.add(R.id.fragment_holder, fragment, "DEVICE_FRAGMENT")
        transaction.addToBackStack(null)
        transaction.commit()
    }
}

And this is the DeviceFragment:

class DeviceFragment : Fragment() {

lateinit var option : Spinner

companion object {
    fun newInstance(): DeviceFragment {
        return DeviceFragment()
    }
}

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    val view = inflater.inflate(R.layout.fragment_device, container, false)
    option = view.spinner

    return inflater?.inflate(R.layout.fragment_device, container, false)
}

}

The fragment_holder just is a FrameLayout.

Thanks in advance