Is something wrong with my layout code using RelativeLayout in xamrin android?

This layout has three controls:two toolbars and a framelayout.I want two toolbars one at the bottom of the layout and another at the top of the layout.The framelayout just lies between them.I have the code like this,but it always has the error saying "No resource found that matches the given name (at 'layout_above' with value '@id/bottomToolBar')".I'm pretty sure I have set the ID of the bottom toolBar. I am so desperate. Many thanks in advance.

<RelativeLayout 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">
<android.support.v7.widget.Toolbar
    android:id="@+id/mainToolBar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:background="@color/colorPrimary"
    android:minHeight="?attr/actionBarSize"
    app:titleTextColor="#FFFFFF"
    app:title="Musics">
</android.support.v7.widget.Toolbar>
<FrameLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@id/mainToolBar"
    android:layout_above="@id/bottomToolBar"
    android:id="@+id/fragmentsContainer"
    />

<android.support.v7.widget.Toolbar
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:id="@+id/bottomToolBar"
    android:minHeight="?attr/actionBarSize"
    android:background="#EFEFEF">
</android.support.v7.widget.Toolbar>
</RelativeLayout>