[gnome-control-center] printers: Fix crash on close

[Marek Kašík <mkasik src gnome org>]

commit 99c9186fd82470c4016f5a7864ec3f1faeb74eb6
Author: Robert Ancell <robert ancell canonical com>
Date:   Sat Jun 27 11:15:02 2020 +1200

    printers: Fix crash on close
    
    It was calling g_free on an object and a warning trying to set a NULL object.

 panels/printers/cc-printers-panel.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/panels/printers/cc-printers-panel.c b/panels/printers/cc-printers-panel.c
index 93cd17f3f..6f18f0dd4 100644
--- a/panels/printers/cc-printers-panel.c
+++ b/panels/printers/cc-printers-panel.c
@@ -323,7 +323,8 @@ cc_printers_panel_dispose (GObject *object)
   free_dests (self);
   g_list_free_full (self->deleted_printers, g_free);
   self->deleted_printers = NULL;
-  g_object_set_data (self->reference, "self", NULL);
+  if (self->reference != NULL)
+    g_object_set_data (self->reference, "self", NULL);
   g_clear_object (&self->reference);
 
   G_OBJECT_CLASS (cc_printers_panel_parent_class)->dispose (object);
@@ -1309,7 +1310,7 @@ cc_printers_panel_init (CcPrintersPanel *self)
 
   g_type_ensure (CC_TYPE_PERMISSION_INFOBAR);
 
-  g_object_set_data_full (self->reference, "self", self, g_free);
+  g_object_set_data_full (self->reference, "self", self, NULL);
 
   builder_result = gtk_builder_add_objects_from_resource (self->builder,
                                                           "/org/gnome/control-center/printers/printers.ui",