back is not working in navigation component

Question

I'm using navigation component and it works to navigate between fragments and the up arrow in the actionbar is shown but a problem occurred that makes the back button is not working also, the up arrow button is not working.

My code in main activity that binds the elements:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    DataBindingUtil.setContentView(this, R.layout.activity_main);
    NavHostFragment navHostFragment = (NavHostFragment) (getSupportFragmentManager().findFragmentById(R.id.nav_host_fragment));
    assert navHostFragment != null;
    NavController navController = navHostFragment.getNavController();
    NavigationUI.setupActionBarWithNavController(this, navController);
}

My main navigation graph code is:

<?xml version="1.0" encoding="utf-8"?>
<navigation 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:id="@+id/main_navigation_graph"
app:startDestination="@id/mainFragment">

<fragment
    android:id="@+id/mainFragment"
    android:name="com.example.jobfinder.screens.fragments.MainFragment"
    android:label="@string/main_fragment_title"
    tools:layout="@layout/fragment_main"
    >
    <action
        android:id="@+id/action_mainFragment_to_employeesFragment"
        app:destination="@id/employeesFragment" />
</fragment>
<fragment
    android:id="@+id/employeesFragment"
    android:name="com.example.jobfinder.screens.fragments.EmployeesFragment"
    android:label="@string/employees_fragment_title"
    tools:layout="@layout/fragment_employees"
    />

My main activitiy xml file:

<?xml version="1.0" encoding="utf-8"?>
<androidx.fragment.app.FragmentContainerView
     android:id="@+id/nav_host_fragment"
     android:name="androidx.navigation.fragment.NavHostFragment"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     app:defaultNavHost="true"
     app:navGraph="@navigation/main_navigation_graph" />

I navigate to the second fragment by observing a LiveDate<Void> object from the fragment like this:

viewModel.cardClicks.observe(getViewLifecycleOwner(),
            __ -> NavHostFragment.findNavController(this).navigate(R.id.action_mainFragment_to_employeesFragment));