Quelle  popup_qrcode.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/qr_popup"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="@dimen/grid_unit_x2">

    <com.google.android.material.card.MaterialCardView
        android:id="@+id/qr_popup_container"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:padding="@dimen/grid_unit_x2"
        app:cardCornerRadius="24dp"
        app:strokeColor="@android:color/white"
        app:strokeWidth="3dp"
        app:cardBackgroundColor="@android:color/white"
        app:cardElevation="0dp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintDimensionRatio="1:1">

        <ImageView
            android:id="@+id/thumbnail_view"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:adjustViewBounds="true"
            android:scaleType="fitCenter"
            android:contentDescription="@string/qr_code" />

    </com.google.android.material.card.MaterialCardView>

</androidx.constraintlayout.widget.ConstraintLayout>