[Vala] Changing label when a button is clicked

[Simon Chan <simon780 gmail com>]

I am writing a simple program that changes the label when a button is
clicked, but I am getting the following error when the button is
clicked:

Gtk-CRITICAL **: gtk_label_set_text: assertion `GTK_IS_LABEL (label)' failed

I am only able to change the label when the label is declared as
"private static Label label"
Help is appreciated, thank you very much!!

Attachment: sample.vala
Description: Text Data