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I'm new at app development and Kotlin and I'm trying to do my first app for learning purposes but I'm kinda lost.
Right now, I have an activity where to log in to my app via firebase. Then what I'm trying to do is a bottom navigation bar where to go between activities. Via some tutorials, I found that the best way to develop is via fragments instead of activities, but I don't know how to work with them.
So right now I have two questions:

1. How to do that when I open my app after login my main activity shows a fragment directly instead after clicking on a button: in the first photo you can see my navigation bar that opens with Home selected but its main activity and it's only when I click again on home that goes to fragment

1. How can I implement buttons inside fragments and go to other activities? For example, on my profile fragment I want to have a log out button that when I click on it goes to my login activity, but it always crashes and gives me this error :
   E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.appilerna, PID: 2263 java.lang.NullPointerException: Attempt to invoke virtual method 'void com.google.android.material.button.MaterialButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at Fragments.ProfileFragment.logOut(ProfileFragment.kt:48) at Fragments.ProfileFragment.onCreateView(ProfileFragment.kt:42) at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2963) at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:518) at androidx.fragment.app.FragmentStateManager.moveToExpectedState(FragmentStateManager.java:282) at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:2189) at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:2100) at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:2002) at androidx.fragment.app.FragmentManager$5.run(FragmentManager.java:524) at android.os.Handler.handleCallback(Handler.java:883) at android.os.Handler.dispatchMessage(Handler.java:100) at android.os.Looper.loop(Looper.java:214) at android.app.ActivityThread.main(ActivityThread.java:7356) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
   My code for main activity:

class MainActivity : AppCompatActivity() {

    private val calendarFragment = CalendarFragment()
    private val favoritesFragment = FavoritesFragment()
    private val profileFragment = ProfileFragment()
    private val homeFragment = HomeFragment()
    private val searchFragment = SearchFragment()

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

        bottom_navigation.setOnItemSelectedListener { it:MenuItem ->
            when (it.itemId) {
                R.id.ic_home -> replaceFragment(homeFragment)
                R.id.ic_search -> replaceFragment(searchFragment)
                R.id.ic_favourites -> replaceFragment(favoritesFragment)
                R.id.ic_calendar -> replaceFragment(calendarFragment)
                R.id.ic_account -> replaceFragment(profileFragment)
            }
            true
        }
    }

    private fun replaceFragment(fragment: Fragment){
        if(fragment != null){
            val transaction = supportFragmentManager.beginTransaction()
            transaction.replace(R.id.fragment_container, fragment)
            transaction.commit()
        }
    }
}

My main activity xml:

<?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=".MainActivity"
    android:background="@color/backgroundColor" >

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/bottom_navigation"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/menu"
        android:background="@color/salmon"
        app:itemIconTint="@color/white"
        app:itemTextColor="@color/white" />

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:id="@+id/fragment_container"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toTopOf="@+id/bottom_navigation"/>

</androidx.constraintlayout.widget.ConstraintLayout>

My ProfileFragment code:

// TODO: Rename parameter arguments, choose names that match
// the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
private const val ARG_PARAM1 = "param1"
private const val ARG_PARAM2 = "param2"

/**
 * A simple [Fragment] subclass.
 * Use the [ProfileFragment.newInstance] factory method to
 * create an instance of this fragment.
 */
class ProfileFragment : Fragment() {
    // TODO: Rename and change types of parameters
    private var param1: String? = null
    private var param2: String? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        arguments?.let {
            param1 = it.getString(ARG_PARAM1)
            param2 = it.getString(ARG_PARAM2)
        }
    }

    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                              savedInstanceState: Bundle?): View? {
        // Inflate the layout for this fragment
        val view = inflater.inflate(R.layout.fragment_profile, container, false)

        logOut(view)

        return view
    }

    fun logOut(view: View){
        logoutButton.setOnClickListener(View.OnClickListener {
            FirebaseAuth.getInstance().signOut()
            //view.context.startActivity(Intent(view.context, LogInActivity::class.java))
            val intent = Intent(activity, LogInActivity::class.java)
            activity?.startActivity(intent)
        })
    }

    companion object {
        /**
         * Use this factory method to create a new instance of
         * this fragment using the provided parameters.
         *
         * @param param1 Parameter 1.
         * @param param2 Parameter 2.
         * @return A new instance of fragment ProfileFragment.
         */
        // TODO: Rename and change types and number of parameters
        @JvmStatic fun newInstance(param1: String, param2: String) =
                ProfileFragment().apply {
                    arguments = Bundle().apply {
                        putString(ARG_PARAM1, param1)
                        putString(ARG_PARAM2, param2)
                    }
                }
    }
}

My profile fragment xml:

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context="Fragments.ProfileFragment">

    <!-- TODO: Update blank fragment layout -->
    <TextView
        android:id="@+id/profile"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Profile"
        android:textSize="36sp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"/>

    <com.google.android.material.button.MaterialButton
        android:id="@+id/logoutButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="160dp"
        android:layout_marginLeft="160dp"
        android:layout_marginRight="163dp"
        android:layout_marginEnd="163dp"
        android:layout_marginBottom="16dp"
        android:text="LOG OUT"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="1.0"
        app:layout_constraintStart_toStartOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>

Thank you

EDIT

I just found answer for my first question doing this on Main Activity:

ft.replace(android.R.id.content, homeFragment)
    ft.commit()

    bottom_navigation.setOnItemSelectedListener { it: MenuItem ->
        when (it.itemId) {
            R.id.ic_home -> replaceFragment(homeFragment)
            R.id.ic_search -> replaceFragment(searchFragment)
            R.id.ic_favourites -> replaceFragment(favoritesFragment)
            R.id.ic_calendar -> replaceFragment(calendarFragment)
            R.id.ic_account -> replaceFragment(profileFragment)
        }
        true
    }
}

private fun replaceFragment(fragment: Fragment){
    if(fragment != null){
        val transaction = supportFragmentManager.beginTransaction()
        transaction.replace(android.R.id.content, fragment)
        transaction.commit()
    }
}

But still having troubles with my profile Logout button.