Flex 4:The default property contents must be contiguous

In Flash Builder 4, when tried to create a MXML component and instantiate it in the main MXML application, I?found that the "Outline" explorer is not displaying the correct tree view anymore, instead it displayed excalmatory marks for each of the components' icon:

And when I?switched to the design view in the MXML editor, it failed to render the preview and showed an error message "The default property contents must be contiguous":

However, when I?compiled and run the application, everything looked OK. I?think this was just an error for the Flash Builder 4 failing to parse and render the outline and preview. The code which was generating such error looks like this:

Note that the "s:layout" from the spark library is also a non-visual component, so it can't be placed between any of the visual components neither, otherwise the same error would occur.

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from:http://www.inluck.net/Blog/Flash-Builder-4-error-The-default-property-contents-must-be-contiguous