[Gambas-user] How to find the source of evil?

[Wolfgang, dl7nb]

Hello!
All works well but I get an error saying:

(MyProgram140708:8024): Gtk-CRITICAL **: gtk_text_buffer_emit_insert: 
assertion `g_utf8_validate (text, len, NULL)' failed

I  think that "140708:8024" is the place from where the faulty call is 
done. But how can I find this/this line in my source code.

Is there a simple solution that I have overlooked?

Thank you for your time!

Wolfgang