I am getting problem to accessing webservice.

Answer 1

It seems strange that you are getting the error on that line as you have already made the check that both result and result._return are not null. It is more likely that the line...

childid.dataProvider = photoFeed; 

...is causing the error because childid is null? This would probably be because you tab navigator has not created the second tab yet. Try adding creationPolicy="all" to the tab navigator to make sure it has been created.

<mx:TabNavigator id="viewstack2"
        selectedIndex="0"
        creationPolicy="all"
        historyManagementEnabled="false"
        width="100%" height="100%">

By default it is set to 'auto' which means it will only create panels other than the default when you click on them.