title: Removing instance after several condition
body: <p>How do i solve the <em>Parameter child must be non-null</em> error?<br>
  I'm trying to remove instances (box1, box2, box3, box4) that have <code>.y &lt;= 56</code></p>
  
  <p>The code follows:</p>
  
  <pre><code>for (i=1; i&lt;=4; i++)
  {
      this["Box" + i].addEventListener(Event.ENTER_FRAME, Move);
  }
  
  function Move(e:Event):void
  {
      e.target.y -=  6;
      if (e.target.y &lt;= 56)
      {
          removeChild(getChildByName(e.target.name));
          Comment.text = "MISS";
          miss +=  1;
          score +=  0;
      }
  }
  </code></pre>