Linearlayout Android: Layout_gravity = "Centre_vertical" fonctionne au moment de la conception mais ne fonctionne pas à l'heure

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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="wrap_content"
    android:layout_gravity="center_vertical"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/dice_image"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        app:srcCompat="@drawable/empty_dice"
        tools:src="@drawable/dice_1" />

    <Button
        android:id="@+id/roll_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:text="@string/roll" />

</LinearLayout>
on design time i got the linearlayout centered verticaly , but when i test it on emulator or my device the linear layout is displayed at the top of the screen