[Vala] compile time errors

["Dr. Amr Osman" <dr3mro gmail com>]

when i try to compile this small app i get these errors
=======
valac helloworld.vala --pkg gtk+-3.0 --pkg glib-2.0
helloworld.vala:20.30-20.33: warning: Argument 1: Cannot pass null to
non-null parameter type
var icons=theme.list_icons(null);
^^^^
/home/dr3mro/Desktop/hellovala/helloworld.vala.c: In function ‘gen_list’:
/home/dr3mro/Desktop/hellovala/helloworld.vala.c:196:9: warning: passing
argument 3 of ‘_vala_string_array_contains’ discards ‘const’ qualifier
from pointer target type [enabled by default]
/home/dr3mro/Desktop/hellovala/helloworld.vala.c:56:17: note: expected
‘gchar *’ but argument is of type ‘const gchar *’
/home/dr3mro/Desktop/hellovala/helloworld.vala.c:208:9: warning: passing
argument 3 of ‘_vala_string_array_contains’ discards ‘const’ qualifier
from pointer target type [enabled by default]
/home/dr3mro/Desktop/hellovala/helloworld.vala.c:56:17: note: expected
‘gchar *’ but argument is of type ‘const gchar *’
Compilation succeeded - 1 warning(s)
========
what is the solution for this ?

Attachment: helloworld.ui
Description: application/designer

Attachment: helloworld.vala
Description: Text Data