A list/grid should have no children declared in XML AdapterViews such as ListViews must be configured with data from Java code, such as a ListAdapter. Issue id: AdapterViewChildren http://developer.android.com/reference/android/widget/AdapterView.html

<?xml version="1.0" encoding="utf-8"?>
<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">

    <ListView
        android:id="@+id/activity_main_lista_de_alunos"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <com.google.android.material.floatingactionbutton.FloatingActionButton
            android:layout_width="89dp"
            android:layout_height="wrap_content" />
    </ListView>
</RelativeLayout>

porque isso está me impossibilitando de mover o floating action button dentro da listview?