[gtk/inspector-layout] inspector: Avoid a critical

[Matthias Clasen <matthiasc src gnome org>]

commit cb3d61bb0e305c759fa429e509eb85eb7752b4a7
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Apr 5 19:44:16 2019 +0000

    inspector: Avoid a critical
    
    If a layout manager does not have layout children,
    quietly hide the layout page.

 gtk/inspector/prop-list.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/gtk/inspector/prop-list.c b/gtk/inspector/prop-list.c
index 60096bbc65..97edae10fc 100644
--- a/gtk/inspector/prop-list.c
+++ b/gtk/inspector/prop-list.c
@@ -528,6 +528,9 @@ gtk_inspector_prop_list_set_layout_child (GtkInspectorPropList *pl,
   if (!layout_manager)
     return;
 
+  if (GTK_LAYOUT_MANAGER_GET_CLASS (layout_manager)->layout_child_type == G_TYPE_INVALID)
+    return;
+
   layout_child = gtk_layout_manager_get_layout_child (layout_manager, GTK_WIDGET (object));
   if (!layout_child)
     return;