avoid warnings if log messages are sent after GTK shutdown

git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@746 bbaffbd6-741e-11dd-a85d-61de82d9cad9

src/pycam/Gui/Project.py

@@ -1404,7 +1404,8 @@ class ProjectGui:
             # remove all non-ascii characters
             message = "".join([char for char in message if ord(char) < 128])
         self.log_model.append((timestamp, title, message))
-        # update the status bar
+        # update the status bar (if the GTK interface is still active)
+        if not self.status_bar.window is None:
             self.status_bar.push(0, message)
 
     @gui_activity_guard