Spaces aren't valid in param names.

--- glade3-3.8.6/gladeui/glade-widget.c	2025-07-08 21:15:46.590965487 +0100
+++ glade3-3.8.6/gladeui/glade-widget.c	2025-07-08 21:15:42.982826331 +0100
@@ -1192,7 +1192,7 @@
 
 	g_object_class_install_property
 		(object_class, 	PROP_SUPPORT_WARNING,
-		 g_param_spec_string ("support warning", _("Support Warning"),
+		 g_param_spec_string ("support-warning", _("Support Warning"),
 				      _("A warning string about version mismatches"),
 				      NULL, G_PARAM_READABLE));