Calling library function with a Pointer(Void) argument

Working on Gtk4 examples using the Gtk4 shard where I call the following method

 def add_constraints_from_description(lines : Enumerable(::String), hspacing : Int32, vspacing : Int32, views : Pointer(Void)) : GLib::List

and specifically the views argument of type Pointer(Void). The code compiles when I pass in Pointer(Void).null for views but than I get a Gtk error.

(crystal-run-main.tmp:12461): Gtk-CRITICAL **: 19:59:39.242: gtk_constraint_layout_add_constraints_from_descriptionv: assertion 'views != NULL' failed

which is expected as I pass in a Pointer(Void).null

The sample code can be found at (removed some of the comments) 

in method constraints

  def constraints()

    vfl = [

    views = Pointer(Void).null
    list = @manager.add_constraints_from_description(vfl,8,8,views)

Crystal 1.9.2
Gtk4 shard 0.15

Without knowing anything about GTK4, from this doc (, it would imply its either a single first view or it’s a null terminated collection of views.