Issue while starting the flex...

I have an app that has been working for over 4 months now, but it started giving me this strange error starting this morning. It doesn't even start the app. Pls let me know if you have any suggestions.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
?at mx.controls::AdvancedDataGridBaseEx/http://www.adobe.com/2006/flex/mx/internal::getHeaderInfo()[C:\work\flex\dmv_automation\projects\datavisualisation\src\mx\controls\Advanc edDataGridBaseEx.as:2366]
?at mx.controls::AdvancedDataGrid/http://www.adobe.com/2006/flex/mx/internal::getHeaderInfo()[C:\work\flex\dmv_automation\projects\datavisualisation\src\mx\controls\Advanc edDataGrid.as:2147]
?at mx.controls::AdvancedDataGrid/getFieldSortInfo()[C:\work\flex\dmv_automation\pr ojects\datavisualisation\src\mx\controls\AdvancedDataGrid.as:6840]
?at mx.controls.advancedDataGridClasses::AdvancedDataGridHeaderRenderer/getFieldSor tInfo()[C:\work\flex\dmv_automation\projects\datavisualisation\src\mx\controls\a dvancedDataGridClasses\AdvancedDataGridHeaderRenderer.as:757]
?at mx.controls.advancedDataGridClasses::AdvancedDataGridHeaderRenderer/measure()[C :\work\flex\dmv_automation\projects\datavisualisation\src\mx\controls\advancedDa taGridClasses\AdvancedDataGridHeaderRenderer.as:481]
?at mx.core::UIComponent/measureSizes()[C:\autobuild\3.2.0\frameworks\projects\fram ework\src\mx\core\UIComponent.as:5956]
?at mx.core::UIComponent/validateSize()[C:\autobuild\3.2.0\frameworks\projects\fram ework\src\mx\core\UIComponent.as:5902]
?at mx.managers::LayoutManager/validateClient()[C:\autobuild\3.2.0\frameworks\proje cts\framework\src\mx\managers\LayoutManager.as:837]
?at mx.controls::AdvancedDataGridBaseEx/calculateHeaderHeight()[C:\work\flex\dmv_au tomation\projects\datavisualisation\src\mx\controls\AdvancedDataGridBaseEx.as:22 49]
?at mx.controls::AdvancedDataGrid/calculateHeaderHeight()[C:\work\flex\dmv_automati on\projects\datavisualisation\src\mx\controls\AdvancedDataGrid.as:3141]
?at mx.controls::AdvancedDataGridBaseEx/makeRowsAndColumns()[C:\work\flex\dmv_autom ation\projects\datavisualisation\src\mx\controls\AdvancedDataGridBaseEx.as:1915]
?at mx.controls::AdvancedDataGrid/makeRowsAndColumns()[C:\work\flex\dmv_automation\ projects\datavisualisation\src\mx\controls\AdvancedDataGrid.as:7169]
?at mx.controls.listClasses::AdvancedListBase/updateDisplayList()[C:\work\flex\dmv_ automation\projects\datavisualisation\src\mx\controls\listClasses\AdvancedListBa se.as:3504]
?at mx.controls::AdvancedDataGridBaseEx/updateDisplayList()[C:\work\flex\dmv_automa tion\projects\datavisualisation\src\mx\controls\AdvancedDataGridBaseEx.as:1778]
?at mx.controls::AdvancedDataGrid/updateDisplayList()[C:\work\flex\dmv_automation\p rojects\datavisualisation\src\mx\controls\AdvancedDataGrid.as:6007]
?at mx.controls.listClasses::AdvancedListBase/validateDisplayList()[C:\work\flex\dm v_automation\projects\datavisualisation\src\mx\controls\listClasses\AdvancedList Base.as:3072]
?at mx.managers::LayoutManager/validateDisplayList()[C:\autobuild\3.2.0\frameworks\ projects\framework\src\mx\managers\LayoutManager.as:622]
?at mx.managers::LayoutManager/doPhasedInstantiation()[C:\autobuild\3.2.0\framework s\projects\framework\src\mx\managers\LayoutManager.as:677]
?at Function/http://adobe.com/AS3/2006/builtin::apply()
?at mx.core::UIComponent/callLaterDispatcher2()[C:\autobuild\3.2.0\frameworks\proje cts\framework\src\mx\core\UIComponent.as:8628]
?at mx.core::UIComponent/callLaterDispatcher()[C:\autobuild\3.2.0\frameworks\projec ts\framework\src\mx\core\UIComponent.as:8568]

Issue while starting the flex...

Maybe it is a timing issue you didn't see before.

Check for areas of the app that might be dependent on other areas of the app being in a certain state, or dependent on data existing.

Or try to boil your case donw to simplified code and post here.

this works