RTE - PopupManager mx.controls.DataGrid

I'm getting a Runtime Exception on Internet Explorer TypeError: Error #1009: Cannot access a property or method of a null object reference. when showing DataGrids on a TitleWindow using PopupManager. It works fine on FireFox. Unfortunately, all our users prefer Internet Explorer.

1) Open a TitleWindow from the main application.Two DataGrids are laid on a TitleWindow vertically.
2) Click into an editable column on the FIRST DataGrid and type something.
3) Move your mouse to another application (For eg., Outlook).
4) Now click into an editable column on the SECOND DataGrid.
5) The following exception is raised and the screen goes BLACK.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at mx.controls::DataGrid/itemEditorItemEditBeginHandler()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/mx/src/mx/controls/DataGrid.as:5287]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/framework/src/mx/core/UIComponent.as:13160]
at mx.controls::DataGrid/commitEditedItemPosition()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/mx/src/mx/controls/DataGrid.as:4115]
at mx.controls::DataGrid/updateDisplayList()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/mx/src/mx/controls/DataGrid.as:1822]
at mx.controls.listClasses::ListBase/validateDisplayList()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/mx/src/mx/controls/listClasses/ListBase.as:3968]
at mx.managers::LayoutManager/validateDisplayList()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/framework/src/mx/managers/LayoutManager.as:744]
at mx.managers::LayoutManager/doPhasedInstantiation()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/framework/src/mx/managers/LayoutManager.as:827]
at mx.managers::LayoutManager/doPhasedInstantiationCallback()[/Users/justinmclean/Documents/ApacheFlexSDK/frameworks/projects/framework/src/mx/managers/LayoutManager.as:1188]
at flash.utils::Timer/_timerDispatch()
at flash.utils::Timer/tick()