How to use YouTube Player Fragment in kotlin?

I'm trying to show some Youtube videos in my Android app. After reading some tutorials, I decided to use YoutubePlayerFragment. However, when I run my code below, I just got some error.

package com.example.signlanguage

import android.os.Bundle
import android.util.Log
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import com.google.android.youtube.player.YouTubeInitializationResult
import com.google.android.youtube.player.YouTubePlayer
import com.google.android.youtube.player.YouTubePlayerFragment
import com.example.signlanguage.databinding.FragmentYoutubeBinding


class youtubeFragment : Fragment(), YouTubePlayer.OnInitializedListener {
    private var _binding: FragmentYoutubeBinding? = null
    private val binding get() = _binding!!
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        _binding = FragmentYoutubeBinding.inflate(inflater, container, false)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val youTubePlayerFragment = childFragmentManager.findFragmentById(R.id.youtube_fragment) as YouTubePlayerFragment
        youTubePlayerFragment.initialize(getString(R.string.api), this)
    }

    override fun onInitializationSuccess(
        provider: YouTubePlayer.Provider?,
        player: YouTubePlayer?,
        wasRestored: Boolean
    ) {
        if(player == null) return
        if(wasRestored) {
            player.play()
        } else {
            player.cueVideo(getString(R.string.video_id))
            player.setPlayerStyle(YouTubePlayer.PlayerStyle.DEFAULT);
        }
    }

    override fun onInitializationFailure(
        p0: YouTubePlayer.Provider?,
        p1: YouTubeInitializationResult?
    ) {
        Log.d("Youtube Player", "Failed to initialize");
    }


}

And this is my layout file:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".youtubeFragment">
    <fragment
        android:name="com.google.android.youtube.player.YouTubePlayerFragment"
        android:id="@+id/youtube_fragment"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        tools:ignore="Instantiatable" />


</androidx.constraintlayout.widget.ConstraintLayout>

Here is the error message I got from the Logcat: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.signlanguage/com.example.signlanguage.MainActivity}: java.lang.NullPointerException: null cannot be cast to non-null type com.google.android.youtube.player.YouTubePlayerFragment

I've try to change val youTubePlayerFragment = childFragmentManager.findFragmentById(R.id.youtube_fragment) as YouTubePlayerFragment youTubePlayerFragment.initialize(getString(R.string.api), this) into val youTubePlayerFragment = childFragmentManager.findFragmentById(R.id.youtube_fragment) as YouTubePlayerFragment? youTubePlayerFragment?.initialize(getString(R.string.api), this), and ended up getting a black video view in my screen. I guess this is because the player is null, but I don't know why this could happen.

Does someone know how to fix this?