Question

In the Scroller.as class line 2139 I’m getting the following error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at spark.components::Scroller/focusInHandler()[E:\dev\4.y\frameworks\projects\spark\src\spark\components\Scroller.as:2139]
    at flash.display::Stage/set focus()

From Scroller.as

/**
 *  @private 
 *  Listens for any focusIn events from descendants 
 */ 
override protected function focusInHandler(event:FocusEvent):void
{
    super.focusInHandler(event);

    // When we gain focus, make sure the focused element is visible
    if (viewport && ensureElementIsVisibleForSoftKeyboard)
    {
        var elt:IVisualElement = focusManager.getFocus() as IVisualElement; 
        lastFocusedElement = elt;
    }
}

Since this is framework code what option do I have to prevent it?

Context
I have created a pop up TitleWindow, added a Module in it and displayed it. The Module has a few States, in each State is a Group, one Group has a List, that List has an ItemRenderer, that ItemRenderer has a Checkbox.

The Module also has a Menu. When the Menu is opened the menu pop up lists the states the Module has available. When an item is selected from the menu pop up I change to another state.

When the state is changed and the last item is the checkbox then the error is generated. At least that’s what I think is happening. I deduced this because in the Scroller class the handler is handling an event. On that event is the current target. That current target is the checkbox.

Update – Steps to reproduce

// inside the Application.mxml
// define variables
public var popup:Group;
public var titleWindow:TitleWindow;


// shows pop up
public function showInspector():void {

    // inside show inspector method
    // create new inspector container
    popup = new InspectorContainer(); // a group implements="mx.managers.IFocusManagerContainer"
    titleWindow = new TitleWindow();
    titleWindow.addElement(popup);

    // display pop up title window
    PopUpManager.addPopUp(titleWindow, this, false);
}

<fx:Declarations>
    <modules:InspectorContainer/>
</fx:Declarations>

1. Show pop up. The pop up is a Title Window with InspectorContainer (a group) as the first element.

2. In pop up change from home state (default state) to online state (this happens when the user clicks a button). The online state has a List. The List has an itemrenderer. The itemrenderer has a checkbox. Select the checkbox. So far so good.

3. The pop up (InspectorContainer) has a mx:MenuBar instance. When you click on an item in the menubar the menubar displays a menu items.

4. Click an item in the menu list. The itemClick menuHandler is called. In this function the pop up changes state.

This is when the error occurs.

Answer 1

Here’s the full code:

<?xml version="1.0" encoding="utf-8"?>
<s:ItemRenderer xmlns:fx="http://ns.adobe.com/mxml/2009" 
            xmlns:s="library://ns.adobe.com/flex/spark" 
            xmlns:mx="library://ns.adobe.com/flex/mx" 
            autoDrawBackground="true"
            width="100%"
            implements="mx.managers.IFocusManagerContainer">

    <fx:Script>
    <![CDATA[
        public function get defaultButton():IFlexDisplayObject {
            return null;
        }

        public function set defaultButton(value:IFlexDisplayObject):void {
            // do nothing
        }

        override public function get systemManager():ISystemManager {
            return null;
        }
    ]]>
    </fx:Script>

    <!-- other stuff -->

    <s:CheckBox id="enabledCheckbox" />

</s:ItemRenderer>

The solution was to implement the IFocusManagerContainer or basically the function it needed. In my case I had to override the systemManager getter in the container of the target object (component) of the focus event in the Scroller focusInHandler handler.

I figured it out this way. The error occurs in the Scroller focusInHandler. The target property of the focus event (event.target) is the Checkbox (the last item to have focus). The container of the Checkbox is my ItemRenderer. This is where I implemented the IFocusManagerContainer interface. I mention this because there are 5 or more parent levels along this path.

In the ItemRenderer I added:

override public function get systemManager():ISystemManager {
    return null;
}

This method is a method you have to implement when you implement the mx.managers.IFocusManagerContainer interface.

That interface also required me to define the defaultButton getter and setter but it was not necessary to solve this error. In tests I removed the interface, removed the defaultButton members and left the systemManager getter and with just that it still solved the problem.

Also, thanks to noobsarepeople2 for help.