gtksourceview gtk_source_view_get_type return 0

I'm not sure if this the correct forum to post, so sorry if
it isn't.

I want to use glib api 'g_type_check_instance_is_a window'
to test if a widget 
is a gtksourceview like this
   g_type_check_instance_is_a (window ,
gtk_source_view_get_type())

but the function 'gtk_source_view_get_type' returns 0. Other
gtk widgets (eg. 
menu_bar_get_type) work properly

Does anyone know how to make 'gtk_source_view_get_type'
work?


thanks
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