[balsa] Do not access a stale pointer

[Peter Bloomfield <peterb src gnome org>]

commit 3759b5298c4221a67aa3baed9766f84820d61ba3
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Wed Feb 8 22:54:51 2017 -0500

    Do not access a stale pointer
    
        * src/main-window.c (balsa_window_destroy): nullify window->preview;
        (bw_action_set_enabled): check for NULL window->preview;
        (bw_idle_replace): ditto.

 ChangeLog         |    8 ++++++++
 src/main-window.c |   25 +++++++++++++++++--------
 2 files changed, 25 insertions(+), 8 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 0b75e88..f52dfa9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2017-02-08  Peter Bloomfield  <pbloomfield bellsouth net>
+
+       Do not access a stale pointer
+
+       * src/main-window.c (balsa_window_destroy): nullify window->preview;
+       (bw_action_set_enabled): check for NULL window->preview;
+       (bw_idle_replace): ditto.
+
 2017-01-24  Peter Bloomfield  <pbloomfield bellsouth net>
 
        Fix some conditional compilation code so that uncrustify does
diff --git a/src/main-window.c b/src/main-window.c
index a950ad1..8467cb1 100644
--- a/src/main-window.c
+++ b/src/main-window.c
@@ -768,7 +768,8 @@ bw_action_set_enabled(BalsaWindow * window,
 {
     GAction *action;
 
-    if (g_object_get_data(G_OBJECT(window), "destroying"))
+    /* Is the window being destroyed? */
+    if (window->preview == NULL)
         return;
 
     action = bw_get_action(window, action_name);
@@ -3088,10 +3089,15 @@ bw_close_mailbox_on_timer(BalsaWindow * window)
 static void
 balsa_window_destroy(GObject * object)
 {
-    g_object_set_data(object, "destroying", GINT_TO_POINTER(TRUE));
-    bw_idle_remove(BALSA_WINDOW(object));
+    BalsaWindow *window;
+
+    window = BALSA_WINDOW(object);
+    bw_idle_remove(window);
+    /* The preview window seems to get finalized without notification;
+     * we no longer need it, so we just drop our pointer: */
+    window->preview = NULL;
 
-    if (G_OBJECT_CLASS(balsa_window_parent_class)->dispose)
+    if (G_OBJECT_CLASS(balsa_window_parent_class)->dispose != NULL)
         G_OBJECT_CLASS(balsa_window_parent_class)->dispose(object);
 
     balsa_unregister_pixmaps();
@@ -4435,10 +4441,13 @@ bw_idle_replace(BalsaWindow * window, BalsaIndex * bindex)
 {
     if (balsa_app.previewpane) {
         bw_idle_remove(window);
-        window->set_message_id =
-            gdk_threads_add_idle((GSourceFunc) bw_idle_cb, window);
-        if (BALSA_MESSAGE(window->preview)->message)
-            gtk_widget_hide(window->preview);
+        /* Skip if the window is being destroyed: */
+        if (window->preview != NULL) {
+            window->set_message_id =
+                gdk_threads_add_idle((GSourceFunc) bw_idle_cb, window);
+            if (BALSA_MESSAGE(window->preview)->message != NULL)
+                gtk_widget_hide(window->preview);
+        }
     }
 }