I ported over a widget from Flex 3 to 4 that used an AdvancedDataGrid and it started getting this error when I added it to a VBOX in ActionScript. It turns out it was missing some of the CSS definitions. See: http://justinimhoff.com/flex-4-and-the-advanceddatagrid-error/.
I copied the CSS code from c:\program files (x86)\Adobe\Adobe Flash Builder 4\sdks\4.1.0\frameworks\projects\datavisualization\data_management.css and added it into the default.css file.
Now it is working. Thought I would let you all know in case anyone else runs into this.
