[#4067] Fix gtk-critical warning when updating username in acount config

sflphone-client-gnome/src/config/accountconfigdialog.c

@@ -259,11 +259,14 @@ static GPtrArray* getNewCredential (GHashTable * properties)
 static void update_credential_cb (GtkWidget *widget, gpointer data UNUSED)
 {
     GtkTreeIter iter;
+
+    if (credentialStore) {
         gtk_tree_model_get_iter_from_string ( (GtkTreeModel *) credentialStore, &iter, "0");
         gint column = GPOINTER_TO_INT (g_object_get_data (G_OBJECT (widget), "column"));
         // g_print ("set password to %s\n", (gchar *) gtk_entry_get_text(GTK_ENTRY(widget)));
         gtk_list_store_set (GTK_LIST_STORE (credentialStore), &iter, column, (gchar *) gtk_entry_get_text (GTK_ENTRY (widget)), -1);
     }
+}
 
 static GtkWidget* create_basic_tab (account_t **a)
 {