Error #2007: Parameter blendMode must be non-null

So you got this error while developing a Flex 3 app.

And you have already consulted this post: http://www.newviewnetworks.com/nvnhome/blog/client/?p=168 who in turn had consulted this post for guidance.

Hmm, even the post by The Agile Tdog refers to another post by Agile UI and this still points to a use case where there is a mismatch between the Flex 3 SDK version you are developing (your project) on and an/some external Flex lib which may have been compiled with a newer or older Flex 3 SDK version.

But that is not my case -- I get this error purely due to my (mis-)use of components in the Flex framework.

Short answer: simply set the blendMode attribute to a valid value on the component that caused the error a la:

blendMode="{BlendMode.NORMAL}"

Synopsis
I was trying to implement reordering of the elements of a control via drag and drop within the same control.

Basically, this is what I did:

<mx:List
 dragEnabled="true"
 dropEnabled="true"
 dragMoveEnabled="true" 
 width="100%"
/>

and then I got the blendMode must not be non-null error.

So my fix was to set the blendMode attribute, which by the way, is inherited from flash.display.DisplayObject.

<mx:List
 dragEnabled="true"
 dropEnabled="true"
 dragMoveEnabled="true" 
        blendMode="{BlendMode.NORMAL}"
 width="100%"
/>

Possible values are listed in the docs here: http://livedocs.adobe.com/flex/3/langref/flash/display/DisplayObject.html#blendMode

[I'm developing using Flex Builder 3 for the Mac which shipped with the Flex 3.2 SDK.]