[epiphany] Fix a runtime critical warning when starting a web application

[Carlos Garcia Campos <carlosgc src gnome org>]

commit ec7b8031151fd9fbea17cd4fb15a9ed7bc025b91
Author: Carlos Garcia Campos <cgarcia igalia com>
Date:   Tue May 28 10:45:11 2019 +0200

    Fix a runtime critical warning when starting a web application
    
    GLib-GObject-CRITICAL **: 09:47:02.477: g_object_bind_property_full: assertion 'G_IS_OBJECT (source)' 
failed
    
    ephy_shell_get_session() returns NULL in web app mode too.

 src/ephy-window.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/src/ephy-window.c b/src/ephy-window.c
index e94e0363d..423d55e3d 100644
--- a/src/ephy-window.c
+++ b/src/ephy-window.c
@@ -3523,6 +3523,7 @@ ephy_window_constructed (GObject *object)
   EphyEmbedShellMode mode;
   EphyWindowChrome chrome = EPHY_WINDOW_CHROME_DEFAULT;
   GApplication *app;
+  EphySession *session;
 
   G_OBJECT_CLASS (ephy_window_parent_class)->constructed (object);
 
@@ -3700,14 +3701,15 @@ ephy_window_constructed (GObject *object)
 
   action_group = gtk_widget_get_action_group (GTK_WIDGET (window), "tab");
   action = g_action_map_lookup_action (G_ACTION_MAP (action_group), "reopen");
-  if (mode == EPHY_EMBED_SHELL_MODE_INCOGNITO || mode == EPHY_EMBED_SHELL_MODE_AUTOMATION) {
-    g_simple_action_set_enabled (G_SIMPLE_ACTION (action), FALSE);
-  } else {
-    g_object_bind_property (G_OBJECT (ephy_shell_get_session (shell)),
+  session = ephy_shell_get_session (shell);
+  if (session) {
+    g_object_bind_property (session,
                             "can-undo-tab-closed",
                             action,
                             "enabled",
                             G_BINDING_SYNC_CREATE);
+  } else {
+    g_simple_action_set_enabled (G_SIMPLE_ACTION (action), FALSE);
   }
 
   window->mouse_gesture_controller = ephy_mouse_gesture_controller_new (window);