ImageView wird nicht angezeigt

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/background_light"
    android:orientation="vertical"
    android:weightSum="1">


    <FrameLayout
        android:id="@+id/frameLayoutGameArena"
        android:layout_width="match_parent"
        android:layout_height="402dp"
        android:layout_weight="0.72">


    </FrameLayout>


</LinearLayout>

public void showDot(){
        screenScale = getResources().getDisplayMetrics().density;
        gameArena = (ViewGroup)findViewById(R.id.frameLayoutGameArena);


        int gameArenaWidth = gameArena.getWidth();
        Log.i("GameActivity","gameArenaWidth = "+ gameArenaWidth);
        int gameArenaHeight = gameArena.getHeight();
        Log.i("GameActivity","gameArenaHeight = "+ gameArenaHeight);




        int dotSize =(int)Math.round(screenScale*50);


        int xAxis = (int)(Math.random()*gameArenaWidth-dotSize);
        int yAxis =  (int)(Math.random()*gameArenaHeight-dotSize);


        ImageView imageViewBlackDot = new ImageView(this);
        imageViewBlackDot.setImageResource(R.drawable.black_dot_50x50);


        FrameLayout.LayoutParams params = new  FrameLayout.LayoutParams(dotSize,dotSize);


        params.leftMargin = xAxis;
        params.topMargin = yAxis;


        params.gravity = Gravity.TOP + Gravity.LEFT;


        gameArena.addView(imageViewBlackDot, params);
    }