title: Access component id of one mxml from another
body: <p>I have two mxml files. one is main that is application tag mxml file and another is my mxml component file. 
  I have a viewstack in my main mxml whose id is, say, "mainViewStack". 
  Now I want to set selectedChild property of "mainViewStack" from my mxml component file. 
  But I m getting error:<b>
  Error #1009: Cannot access a property or method of a null object reference.</b>
   on accessing mainObj.mainViewStack.selectedChild.id where mainObj is the object of main mxml file.
  Please help me out.
  Thank u.</p>