Android : How can I access to a view programmatically that declare invisible in xml?

I have a layout like this:

<RelativeLayout 
            android:id="@+id/testLayout"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_marginBottom="64dp"
            >
            <ImageButton 
                android:layout_width="32dp"
                android:layout_height="32dp"
                android:layout_alignParentLeft="true"
                android:layout_marginLeft="50dp"/>
            <ImageButton 
                android:layout_width="32dp"
                android:layout_height="32dp"
                android:layout_alignParentLeft="true"
                android:layout_marginLeft="100dp"
                android:visibility="invisible"/>
</RelativeLayout>

Now I want to acces the second button in java code, so this is the code section that cause NullPointerException:

RelativeLayout rl = (RelativeLayout) findViewById(R.id.testLayout);
ImageButton b = (ImageButton) rl.getChildAt(1);