FAB on top of Bottom Navigation

How to display a fab on top of a Bottom Navigation? This is what I tried:

<com.google.android.material.floatingactionbutton.FloatingActionButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="@+id/trendingPosts"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/trendingPosts"
        android:focusedByDefault="true"
        android:focusable="true"/>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottom_menu"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="@+id/trendingPosts"
        app:layout_constraintTop_toBottomOf="@+id/trendingPosts"
        app:menu="@menu/main_menu"
        android:focusable="false"
        android:focusedByDefault="false"/>

But, the fab stays below it. See the image:

A good hack is to use elevation parameter in both BottomNavigationView and FloatingActionButton and put elevation of fab button more than BottomNavigationView like this :

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout  
xmlns:app="http://schemas.android.com/apk/res-auto"      
xmlns:tools="http://schemas.android.com/tools"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottom_nav_instructor"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom"
    android:background="@color/white"
    android:elevation="15dp"
    app:itemIconSize="18dp"
    app:itemTextColor="@color/light_grey_text_color"
    style="@style/Theme.OpenInApp"
    app:itemIconTint="@color/light_grey_text_color"
    app:labelVisibilityMode="labeled"
    app:layout_constraintBottom_toBottomOf="parent"
    app:menu="@menu/home_bottom_nav">

</com.google.android.material.bottomnavigation.BottomNavigationView>

<com.google.android.material.floatingactionbutton.FloatingActionButton
    android:id="@+id/fab"
    android:layout_marginBottom="25dp"
    android:layout_gravity="bottom|right"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:src="@drawable/add"
    app:elevation="20dp"
    app:layout_constraintBottom_toBottomOf="@+id/bottom_nav_instructor"
    app:layout_constraintEnd_toEndOf="@+id/bottom_nav_instructor"
    app:layout_constraintStart_toStartOf="@+id/bottom_nav_instructor" />

 </androidx.constraintlayout.widget.ConstraintLayout>