[flexcoders] Getting a the supplied DisplayObject must be a child of the caller error

[lynnkuh]

I'm using the Flex MDI Explorer and instead of using 

<flexmdi:MDICanvas id="mdiCanvas" horizontalScrollPolicy="off"
verticalScrollPolicy="off"
                        width="100%" height="100%" backgroundColor="#FFFFFF"
backgroundAlpha="0">
                        <flexmdi:MDIWindow id="win1"
                                title="Window 1 has a really really long title 
to show support for
truncation"
                                x="30" y="30" width="400" height="200" />
                        <flexmdi:MDIWindow id="win2"
                                title="Window 2"
                                x="450" y="30" width="250" height="300">
                                <ns:SampleContent id="sample" height="100%" 
width="100%" />
                        </flexmdi:MDIWindow>
</flexmdi:MDICanvas>

I created an actionscript variable mdiCanvas. I'm getting the error
when I try and minimize the window, close the window, or make it full
size after calling the addWindow function. Here's my addWindow function:

private function addWindow():void
                        {
                                var win:MDIWindow = new MDIWindow();
                                win.width = 350;
                                win.title = "Window " +
String(mdiCanvas.windowManager.windowList.length + 1);
                                mdiCanvas.windowManager.add(win);
                                win.addChild(new SampleContent());
                                addChild(win);
                        }

How can I get the windows to work? I don't want the MXML windows open
when I first start the application which is the default.