[Vala] Refresh TextBuffer in Vala

[simargl <archpup gmail com>]

Hi,

To learn more about Vala programming language, I wanted to make a very
simple text editor something like leafpad in Vala and Gtk+3.

Source code added here http://pastebin.com/q3x4rfmL

Problem is when I try to make or open new file, that new text_buffer fails
to show in text_view with this error.

(editor:4282): Gtk-CRITICAL **: gtk_text_buffer_set_text: assertion
`GTK_IS_TEXT_BUFFER (buffer)' failed

Any ideas how to fix that problem?

Kind regards