[geary/cherry-pick-737bdba1] ComposerWidget: Don't add invalid Message-ID to In-Reply-To

[Michael Gratton <mjog src gnome org>]

commit e40a78f0dfd19a1369416942fe61edb268c5d867
Author: Michael Gratton <mike vee net>
Date:   Fri Nov 1 15:39:35 2019 +1100

    ComposerWidget: Don't add invalid Message-ID to In-Reply-To
    
    Although Message-ID is usually present, it may not be. If not, the
    composer should not be added a null id to the In-Reply-To field.

 src/client/composer/composer-widget.vala | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/client/composer/composer-widget.vala b/src/client/composer/composer-widget.vala
index 31a0dff8..3df0446b 100644
--- a/src/client/composer/composer-widget.vala
+++ b/src/client/composer/composer-widget.vala
@@ -1159,8 +1159,10 @@ public class ComposerWidget : Gtk.EventBox, Geary.BaseInterface {
             this.to_entry.modified = this.cc_entry.modified = false;
         }
 
-        in_reply_to.add(referred.message_id);
-        referred_ids.add(referred.id);
+        if (referred.message_id != null) {
+            this.in_reply_to.add(referred.message_id);
+        }
+        this.referred_ids.add(referred.id);
     }
 
     public CloseStatus should_close() {