Try re-initializing the chat window each time it's re-opened after being disposed.

src/net/java/sip/communicator/impl/gui/main/chat/ChatWindowManager.java

@@ -692,8 +692,8 @@ private void disposeChatWindow()
         }
 
         ContactList clist
-            = chatWindow.getMainFrame()
-                .getContactListPanel().getContactList();
+            = mainFrame.getContactListPanel().getContactList();
+
         ContactListModel clistModel
             = (ContactListModel) clist.getModel();