[Gtk-sharp-list] button label -> nullpointer exception

[Michael]

Hi,

a calendar popup window (with a okay button) should the user let choose
a data.
The data recieved from the popup window should be shown on an other
button (show). Instead of a label change a
"System.NullReferenceException" is thrown.
Which is a little rare, because all widgets which were build with glade,
are defined in the .cs file.

Here the code snip: