I solved the problem. (Or found a workaround)
I moved the complete initialization stuff out of the constructor of the MyLinearLayout. If I then adding a View after the layout has been completely generated, everything works.
Like this:
MyLinearLayout ll = new MyLinearLayout(getContext());
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(100, 100);
ll.setLayoutParams(params);
ll.setBackgroundColor(Color.RED);
ImageView v = new ImageView(getContext());
params = new LinearLayout.LayoutParams(50, 50);
v.setLayoutParams(params);
v.setBackgroundColor(Color.GREEN);
ll.addView(v);
addView(ll);
I don't know why the other way doesn't work. Thanks for the fast answers!