Android : RelativeLayout: layout_alignWithParentIfMissing not working

I am having a RelativeLayout with two TextView objects. I want @+id/b to move up if @+id/a is missing.

I therefore tried to set the content null when I inflate the parent view but all what happens is that there is just no text displayed. @+id/b remains on its place and @+id/a is still there - without text.

public View getView(int position, View convertView, ViewGroup parent) {
    // ...
    holder.a.setText(null);
    // ...
}

I've also tried to remove with parent.removeView(holder.a) resulting in a first class app-crash.

Where did I go wrong?

<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
    android:orientation="vertical">    
    <TextView
        android:id="@+id/a"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="14sp"/>    
    <TextView
        android:id="@+id/b"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="14sp"
        android:layout_alignWithParentIfMissing="true"
        android:layout_below="@+id/a"
        android:layout_alignParentBottom="@+id/a"/>
</RelativeLayout>