Getting error while maintaining flash code

I am new to flashbuilder. I have got this existing code to maintain. All of the mxml files have outermost tag s:WindowedApplication. The navigation from one page to another is done using code as follows:

public function help_clickHandler(event:MouseEvent):void
{
    var dTracker:aboutProduct = new aboutProduct();
    this.addElement(dTracker);
}

After navigating to a new page I get following error on clicking any where on newly loaded page:

ArgumentError: Error #2025: The supplied DisplayObject must be a child of the caller.
  at flash.display::DisplayObjectContainer/getChildIndex()
  at mx.managers::SystemManager/getChildIndex()  [E:\dev\4.y\frameworks\projects\framework\src\mx\managers\SystemManager.as:1823]
  at mx.managers.systemClasses::ActiveWindowManager/mouseDownHandler()  [E:\dev\4.y\frameworks\projects\framework\src\mx\managers\systemClasses\ActiveWindowManager.as:483]
  at flash.events::EventDispatcher/dispatchEventFunction()
  at flash.events::EventDispatcher/dispatchEvent()
  at mx.core::UIComponent/dispatchEvent()[E:\dev\4.y\frameworks\projects\framework\src\mx\core\UIComponent.as:13152]
  at mx.managers::SystemManager/mouseEventHandler()  [E:\dev\4.y\frameworks\projects\framework\src\mx\managers\SystemManager.as:2918]


Error: Error #3003: File or directory does not exist.
    at flash.filesystem::File/copyTo()
    at studyTopics/studytopic_changeHandler()[C:\flash\46\HondaLMS\src\studyTopics.mxml:81]
    at studyTopics/__lst_change()[C:\flash\46\HondaLMS\src\studyTopics.mxml:136]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\4.y\frameworks\projects\framework\src\mx\core\UIComponent.as:13152]
    at spark.components::List/commitSelection()[E:\dev\4.y\frameworks\projects\spark\src\spark\components\List.as:1278]
    at spark.components::List/commitProperties()[E:\dev\4.y\frameworks\projects\spark\src\spark\components\List.as:1148]
    at mx.core::UIComponent/validateProperties()[E:\dev\4.y\frameworks\projects\framework\src\mx\core\UIComponent.as:8219]
    at spark.components::List/item_mouseDownHandler()[E:\dev\4.y\frameworks\projects\spark\src\spark\components\List.as:1915]
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()[E:\dev\4.y\frameworks\projects\framework\src\mx\core\UIComponent.as:13152]
    at mx.managers::SystemManager/mouseEventHandler()[E:\dev\4.y\frameworks\projects\framework\src\mx\managers\SystemManager.as:2918]

Please help me solve this runtime error.

Update:

There is a technical error in the design of application, i.e., all of the mxml have WindowApplication. There should be only one WindowApplication and rest of them should be Application type (reference). Still I have to figure out, how to navigate from one page to another page and return back to main page or previous page.

In one flex application there can be only one mxml with WindowApplication container. Rest of the containers should be Group.

I left landing page as WindowApplication and changed rest in Group, this revolved the issue I was facing.