Troubleshooting PopUpManager.createPopUp

I have an intermittent bug where a custom titlewindow created by PopUpManager.createPopUp sometimes has trouble completing. It doesn't always happen, but it happens often enough that I suspect it has a predictable cause, I'm just not seeing what it is.

Here's the scenario:

A function calls PopUpManager.createPopUp with a custom TitleWindow.
According to trace, the custom TitleWindow is initialized, but doesn't reach "creationComplete" until PopUpManager.createPopUp is called again (say, by clicking on a button that opens a different pop-up).

What are the possible reasons PopUpManager might be unable to complete creation of a pop-up?

Here's the MXML for the pop-up. I've stripped it down to the bare bones and it still happens, so I suspect the problem lies elsewhere:

Code:

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<?xml version="1.0" encoding="utf-8"?>
<mx:TitleWindow xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" width="450" height="100%" close="close();" initialize="onInit();" creationComplete="onCreationComplete();" showCloseButton="false"  styleName="helpDialog" horizontalScrollPolicy="off">
        <mx:Script>
                <![CDATA[
                        import mx.managers.PopUpManager;
                       

                        private function onInit():void
                        {
                                trace("Initialized");
                        }
                       
                        private function onCreationComplete():void
                        {
                                trace("Creation Complete");
                                this.y = 485 - this.height;
                                this.x = 15;
                        }

                        public function close():void
                        {
                                PopUpManager.removePopUp(this);
                                this.dispatchEvent(new Event("onClose"));
                        }
                ]]>
        </mx:Script>
        <mx:HBox id='missionContents' styleName='helpContents' width="95%">
               
        </mx:HBox>
        <mx:ControlBar paddingTop="5" paddingBottom="5">
                        <mx:Spacer width="100%"/><mx:Button label="OK" height="20" click="close();"/>
        </mx:ControlBar>
</mx:TitleWindow>

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