The guards weren't working because we were accessing data inside the
arguments before checking them. Postpone data access until after the
guards.

0x00007ffff741203c in gtk_box_pack (box=box@entry=0x0, child=0xe0b1b0, expand=expand@entry=0, fill=fill@entry=0, padding=padding@entry=0, pack_type=pack_type@entry=GTK_PACK_START)
    at /usr/src/debug/gtk3-3.24.28-2.fc34.x86_64/gtk/gtkbox.c:1530
1530	  GtkBoxPrivate *private = box->priv;
(gdb) bt
 #0  0x00007ffff741203c in gtk_box_pack (box=box@entry=0x0, child=0xe0b1b0, expand=expand@entry=0, fill=fill@entry=0, padding=padding@entry=0, pack_type=pack_type@entry=GTK_PACK_START)
     at /usr/src/debug/gtk3-3.24.28-2.fc34.x86_64/gtk/gtkbox.c:1530
 #1  0x00007ffff741223c in gtk_box_pack_start (box=box@entry=0x0, child=<optimized out>, expand=expand@entry=0, fill=fill@entry=0, padding=padding@entry=0)
     at /usr/src/debug/gtk3-3.24.28-2.fc34.x86_64/gtk/gtkbox.c:2179