[glade] GladeProject: show message on invalid GtkBuilder file

[Juan Pablo Ugarte <jpu src gnome org>]

commit cabb5bebe3159e81a2e7e4237aaf90700023aa3d
Author: Juan Pablo Ugarte <juanpablougarte gmail com>
Date:   Thu Jun 18 18:58:29 2020 -0300

    GladeProject: show message on invalid GtkBuilder file
    
    Show user an error dialog if he tries to open an old
    libglade file.
    
    https://gitlab.gnome.org/GNOME/glade/-/issues/389

 gladeui/glade-project.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/gladeui/glade-project.c b/gladeui/glade-project.c
index 132b6deb..9f474c2d 100644
--- a/gladeui/glade-project.c
+++ b/gladeui/glade-project.c
@@ -2127,8 +2127,14 @@ glade_project_load_internal (GladeProject *project)
 
   if (!glade_xml_node_verify_silent (root, GLADE_XML_TAG_PROJECT))
     {
-      g_warning ("Couldnt recognize GtkBuilder xml, skipping %s",
-                 load_path ? load_path : priv->path);
+      if (glade_xml_node_verify_silent (root, "glade-interface"))
+        glade_util_ui_message (glade_app_get_window (), GLADE_UI_ERROR, NULL,
+                               "This version of Glade does not support old libglade files.\n"
+                               "Please use Glade 3.8 for GTK 2 files.");
+      else
+        glade_util_ui_message (glade_app_get_window (), GLADE_UI_ERROR, NULL,
+                               "Couldn't recognize GtkBuilder xml.\nskipping %s",
+                               load_path ? load_path : priv->path);
       glade_xml_context_free (context);
       g_free (load_path);
       priv->loading = FALSE;