If we cancel the update, we must clear all the pointers too

mitterlib/ui/ui_pygtk.py

@@ -662,6 +662,11 @@ class Interface(object):
     def _clear_text(self, widget):
         """Clear the text field."""
         self._update_text.get_buffer().set_text('')
+        
+        self._delete_iter = None
+        self._reply_message_id = None
+        self._repost_message_id = None
+
         return
 
     def _quit_app(self, widget=None, user_data=None):