If the property might be read before the loading has completed, then _positionArray should be initialized in the constructor to a new (empty) Array. I have no clue how your using this, but my guess is that you probably want to use data binding to notify of a change to the array.
Try to dispatch an event in your ActionScript class when the setPositionArray(e:ContentEvent):void function in your AS class is called and listen for that event in your file where you are doing trace.
I mean dispatch an event when your _positionArray is poupulated in your AS file(Which is populated in set method positionArray() which is called after the below function in your AS file is executed...
public function setPositionArray(e:ContentEvent):void
positionArray = e.param.data as Array;
public function set positionArray(arr:Array):void
_positionArray = arr;
So after doing this listen for an event in your file where you want to access the array variable.
private function onContentLoadComplete(event:Event)
If this post answers your question or helps, please kindly mark it as such.