[flexcoders] addchild error

[wifi19]

I am having some trouble with addchild

  var   thePrintView:FormPrintView = new FormPrintView();
                                
                //myHbox.addChild(thePrintView); 
        var _caPan:Canvas = new Canvas();
         
          _caPan.height=300;
          _caPan.width = 200;
 
        
    _caPan.addChild(thePrintView);
    _caPan.addChild(thePrintView);

second addchild generate a error

RangeError: Error #2006: L'index indiqué sort des limites.
        at flash.display::DisplayObjectContainer/getChildAt()
        at mx.core::Container/getChildAt()
        at mx.core::UIComponent/set document()
        at 
mx.core::UIComponent/http://www.adobe.com/2006/flex/mx/internal::addin
gChild()
        at 
mx.core::Container/http://www.adobe.com/2006/flex/mx/internal::addingC
hild()
        at mx.core::Container/addChildAt()
        at mx.core::Container/addChild()
        at MyLoginForm/process()
        at MyLoginForm/___TitleWindow1_initialize()
        at 
flash.events::EventDispatcher/flash.events:EventDispatcher::dispatchEv
entFunction()
        at flash.events::EventDispatcher/dispatchEvent()
        at mx.core::UIComponent/dispatchEvent()
        at mx.core::UIComponent/set processedDescriptors()
        at mx.core::Container/createComponentsFromDescriptors()
        at mx.containers::Panel/createComponentsFromDescriptors()
        at mx.core::Container/mx.core:Container::createChildren()
        at mx.containers::Panel/mx.containers:Panel::createChildren()
        at mx.core::UIComponent/initialize()
        at mx.core::Container/initialize()
        at MyLoginForm/initialize()
        at 
mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal:
:childAdded()
        at 
mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal:
:rawChildren_addChildAt()
        at mx.managers::SystemManager/addChild()
        at mx.managers::PopUpManagerImpl/addPopUp()
        at mx.managers::PopUpManager$/addPopUp()
        at print/doPrint()
        at print/__printDG_click()