[gnome-contacts] ContactPane: correctly set this.sheet as nullable.

[Niels De Graef <nielsdg src gnome org>]

commit c15ea8360df5a87798a34e150a9a131280912cdd
Author: Niels De Graef <nielsdegraef gmail com>
Date:   Mon Mar 26 07:43:51 2018 +0200

    ContactPane: correctly set this.sheet as nullable.

 src/contacts-contact-pane.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/contacts-contact-pane.vala b/src/contacts-contact-pane.vala
index 484a9f4..d0fa95e 100644
--- a/src/contacts-contact-pane.vala
+++ b/src/contacts-contact-pane.vala
@@ -42,7 +42,7 @@ public class Contacts.ContactPane : Stack {
   private ScrolledWindow contact_sheet_page;
   [GtkChild]
   private Container contact_sheet_container;
-  private ContactSheet sheet;
+  private ContactSheet? sheet = null;
 
   [GtkChild]
   private Box contact_editor_page;