Draw a circle onto a view (android)

I am starting with my first attempts to write an android app. I'd like to to visualize the Monte-Carlo-Approximation for pi. Hence I first want to draw a Circle onto a view but i dont get it working! I have tried to create my own "CircleView" Class which extends "View" and overwrite the onDraw(..) method like its explained over here: How to draw circle by canvas in Android?

This is my CircleView Class

public class CircleView extends View {
    public CircleView(Context context) {
        super(context);
    }

    protected void onDraw(Canvas canvas){
        super.onDraw(canvas);
        Paint paint = new Paint();
        paint.setColor(150);
        canvas.drawCircle(50,50,20,paint);
    }
}

I have inserted the CircleView into a LinearLayout with the follow XML-code

<com.tak3r07.montecarlopi.CircleView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/circleView"
    android:layout_weight="1"/>

(Btw Android Studio is telling me in the XML-view at the right side: "Rendering Problems Custom view CircleView is not using the 2- or 3-argument View constructors; XML attributes will not work")

The App just crashes with the following log: http://ift.tt/1DqNRIc

Can someone tell what i did wrong?

I thought this setup would create an activity with a view which displays a circle.

Regards