setSupportActionBar()

can only be called with toolbar. I’ve just wrapped the toolbar with the BottomAppBar.

<android.support.design.bottomappbar.BottomAppBar
    android:id="@+id/bottom_appbar"
    android:layout_width="match_parent"
    android:layout_height="56dp"
    android:layout_gravity="bottom"
    app:backgroundTint="@color/colorPrimary"
    app:fabAttached="true"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
    app:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

</android.support.design.bottomappbar.BottomAppBar>