add: if (!selectedMessages) return false;  
context: function BeginDragThreadPane(event){    debugDump("BeginDragThreadPane\n");    var threadTree = GetThreadTree();    var selectedMessages = GetSelectedMessages();    //A message can be dragged from one window and dropped on another window    //therefore setNextMessageAfterDelete() here     //no major disadvantage even if it is a copy operation    SetNextMessageAfterDelete();    return BeginDragTree(event, threadTree, selectedMessages, "text/x-moz-message");}
meta: 9421	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/9421/52630693e2db3b06cd933bd321a2e34ed48d975e/messengerdnd.js/buggy/mailnews/base/resources/content/messengerdnd.js