Fixes NPE, do nothing and close chat invite dialog if nothing selected.

src/net/java/sip/communicator/impl/gui/main/chat/conference/ChatInviteDialog.java

@@ -127,8 +127,11 @@ private void inviteContacts()
         Collection<String> selectedContactAddresses = new ArrayList<String>();
 
         // Obtain selected contacts.
-        Iterator<UIContact> selectedContacts
-            = destContactList.getContacts(null).iterator();
+        Collection<UIContact> contacts = destContactList.getContacts(null);
+        if(contacts == null)
+            return;
+
+        Iterator<UIContact> selectedContacts = contacts.iterator();
 
         if (selectedContacts != null)
         {