[geary/wip/538-crash-changing-from] Fix crash when changing composer From address

[Michael Gratton <mjog src gnome org>]

commit a52ff364542eb38c63d790bed17ec779c073a306
Author: Michael Gratton <mike vee net>
Date:   Sat Aug 17 15:55:35 2019 +1000

    Fix crash when changing composer From address
    
    Fixes #538

 src/client/composer/composer-widget.vala | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/src/client/composer/composer-widget.vala b/src/client/composer/composer-widget.vala
index bbe99190..e34736e4 100644
--- a/src/client/composer/composer-widget.vala
+++ b/src/client/composer/composer-widget.vala
@@ -1398,7 +1398,11 @@ public class ComposerWidget : Gtk.EventBox, Geary.BaseInterface {
         }
 
         GLib.Cancellable internal_cancellable = new GLib.Cancellable();
-        cancellable.cancelled.connect(() => { internal_cancellable.cancel(); });
+        if (cancellable != null) {
+            cancellable.cancelled.connect(
+                () => { internal_cancellable.cancel(); }
+            );
+        }
         this.draft_manager_opening = internal_cancellable;
 
         Geary.App.DraftManager new_manager = new Geary.App.DraftManager(account);