[balsa] main-window: Avoid a crash

[Peter Bloomfield <peterb src gnome org>]

commit 774ba0e507c45ba2f4fd7047bbe8ef353880b381
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Wed May 6 17:59:30 2020 -0400

    main-window: Avoid a crash
    
    caused by the previous commit.

 src/main-window.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/main-window.c b/src/main-window.c
index 5cdd3147b..05b6cf404 100644
--- a/src/main-window.c
+++ b/src/main-window.c
@@ -4631,7 +4631,7 @@ balsa_window_decrease_activity(BalsaWindow * window, const gchar * message)
     const gchar *new_message = NULL;
     gboolean clear_fraction = FALSE;
 
-    if (priv->progress_bar == NULL)
+    if (priv->progress_bar == NULL || priv->activity_messages == NULL)
         return;
 
     link = g_slist_find_custom(priv->activity_messages, message,