Dispatching GridSelectionEvent throw exception at runtime - A build issue with FB 4.5

Question

This is a strang probelm I am facing with.
My code dispatches GridSelectionEvent after setting girds' selected item in one logic path. Following is code lines -
myDataGrid.selectedItem = instanceTag;
myDataGrid.dispatchEvent(new GridSelectionEvent(GridSelectionEvent.SELECTION_CHANGE));
Flash player throws following error on above statements encounter -
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at spark.accessibility::DataGridAccImpl/eventHandler()
    at flash.events::EventDispatcher/dispatchEventFunction()
    at flash.events::EventDispatcher/dispatchEvent()
    at mx.core::UIComponent/dispatchEvent()
I am saying it is a strang for me because when I use swf file generated by Flex Builder (4.5)'s export to release build, it does not throw error and code works fine as expected.
But, when I user swf file generated by Ant script, I face the above issue.
The Flex builder settings is as shown in following image -
Ant  mxmlc script is as follow -
<mxmlc      
           file="${basedir}/src/AppMonConfigWizard.mxml"
            static-rsls="false"
           keep-generated-actionscript="false"
           output="${basedir}/bin-release/AppMonConfigWizard.swf" >
     <locale>en_US</locale>
      <load-config filename="${FLEX_HOME}/frameworks/flex-config.xml"/>
      <source-path path-element="${FLEX_HOME}/frameworks"/>
      <source-path path-element="${basedir}/src/locale/{locale}"></source-path>
      <compiler.library-path file="${basedir}/../wizard_framework/bin/wizard_framework.swc" append="true"/>
     <compiler.library-path file="${basedir}/../../utils/bin/utils.swc" append="true"/>
      <include-resource-bundles>ErrorMessages</include-resource-bundles>
      <include-resource-bundles>Strings</include-resource-bundles>
</mxmlc>
As mentioned earlier that following statements cause exception -
     myDataGrid.selectedItem = instanceTag;
     myDataGrid.dispatchEvent(new GridSelectionEvent(GridSelectionEvent.SELECTION_CHANGE));
If I change it to
     myDataGrid.setSelectedIndex(<some index value>)   /*selectedItem replaced with setSelectedIndex*/
     myDataGrid.dispatchEvent(new GridSelectionEvent(GridSelectionEvent.SELECTION_CHANGE));
it does not throw exception.  I would like to mention that myDataGrid.selectedItem = instanceTag; does sets the selected item but then in that case issue arises when dispatching event.
I my application, similar statemetns were lot at many places (as there were no such issue with SDK 3.1 which was using earlier) so instead of making changes there, it would be better to imitate the build that Flex builder uses when exported the release build.
Thanks in advance,
Prithveesingh Zankat.

Answer 1

Solved it: had set the only public surface contracts checkbox by mistake and the cosntructor was protected...
Luis Abreu