[shotwell] Fix critical when re-using popup menu

[Jens Georg <jensgeorg src gnome org>]

commit fdf099ed0902a2b7553ee78b17a548e1b560f8de
Author: Jens Georg <mail jensge org>
Date:   Fri Dec 16 23:11:14 2016 +0100

    Fix critical when re-using popup menu
    
    Signed-off-by: Jens Georg <mail jensge org>

 src/sidebar/Tree.vala |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/src/sidebar/Tree.vala b/src/sidebar/Tree.vala
index 5ba28fa..9c80f00 100644
--- a/src/sidebar/Tree.vala
+++ b/src/sidebar/Tree.vala
@@ -845,7 +845,9 @@ public class Sidebar.Tree : Gtk.TreeView {
         if (context_menu == null)
             return false;
 
-        context_menu.attach_to_widget (this, null);
+        if (context_menu.get_attach_widget() == null) {
+            context_menu.attach_to_widget (this, null);
+        }
 
         if (event != null)
             context_menu.popup(null, null, null, event.button, event.time);