[balsa/gtk4: 218/294] mime-widget-multipart: Build with gtk4

[Peter Bloomfield <peterb src gnome org>]

commit afcb91813842932ca35eef39992d89d6ae2d4005
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Fri Oct 23 13:48:56 2020 -0400

    mime-widget-multipart: Build with gtk4

 src/balsa-mime-widget-multipart.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/balsa-mime-widget-multipart.c b/src/balsa-mime-widget-multipart.c
index fbaeb001d..e5cfb6ac7 100644
--- a/src/balsa-mime-widget-multipart.c
+++ b/src/balsa-mime-widget-multipart.c
@@ -53,7 +53,7 @@ balsa_mime_widget_new_multipart(BalsaMessage * bm,
                                           mime_body->was_encrypted, FALSE, NULL);
 
         mw = g_object_new(BALSA_TYPE_MIME_WIDGET, NULL);
-        gtk_container_add(GTK_CONTAINER(mw), crypto_frame);
+        gtk_box_append(GTK_BOX(mw), crypto_frame);
     }
 
     balsa_mime_widget_set_container(mw, widget);