[shotwell/shotwell-0.26] Fix crash when no import plugin is enabled

[Jens Georg <jensgeorg src gnome org>]

commit e54216f18f6984a78c9a07f1b150a1c72cfc49a1
Author: Jens Georg <mail jensge org>
Date:   Thu Jun 1 18:42:47 2017 +0200

    Fix crash when no import plugin is enabled
    
    https://bugzilla.gnome.org/show_bug.cgi?id=783250

 src/data_imports/DataImportsUI.vala |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/src/data_imports/DataImportsUI.vala b/src/data_imports/DataImportsUI.vala
index 5b94dcb..2c0cd40 100644
--- a/src/data_imports/DataImportsUI.vala
+++ b/src/data_imports/DataImportsUI.vala
@@ -295,7 +295,9 @@ public class DataImportsDialog : Gtk.Dialog {
             close_cancel_button = new Gtk.Button.with_mnemonic("_Cancel");
             close_cancel_button.set_can_default(true);
             ((Gtk.HeaderBar) get_header_bar()).pack_start(close_cancel_button);
-            ((Gtk.HeaderBar) get_header_bar()).pack_end(service_selector_box);
+            if (service_selector_box != null) {
+                ((Gtk.HeaderBar) get_header_bar()).pack_end(service_selector_box);
+            }
         }
         else {
             add_button (_("_Cancel"), Gtk.ResponseType.CANCEL);