[gnome-tweaks] Fix assertion error on startup

[Jeremy Bicha <jbicha src gnome org>]

commit 5046c5a188109009ebdcfe2d2392e60137a3732d
Author: Çağatay Yiğit Şahin <cyigitsahin outlook com>
Date:   Wed Aug 7 00:05:43 2019 +0300

    Fix assertion error on startup
    
    Previously the app tried to sync the visible part of titlebar
    to main box before setting the visible widget in main box. This
    commit fixes that.
    
    (as mentioned in https://gitlab.gnome.org/GNOME/gnome-tweaks/merge_requests/40#note_573943)

 gtweak/tweakview.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)
---
diff --git a/gtweak/tweakview.py b/gtweak/tweakview.py
index 30a4f13..5de11d6 100644
--- a/gtweak/tweakview.py
+++ b/gtweak/tweakview.py
@@ -33,7 +33,6 @@ class Window(Gtk.ApplicationWindow):
 
         self.menu_btn = Gtk.MenuButton()
         titlebar = self.titlebar()
-        self.main_box.bind_property("visible-child-name", titlebar, "visible-child-name", 
GObject.BindingFlags.SYNC_CREATE)
         self.set_titlebar(titlebar)
         self._update_decorations()
 
@@ -43,6 +42,9 @@ class Window(Gtk.ApplicationWindow):
         self.main_box.add(right_box)
         self.main_box.child_set(right_box, name="content")
 
+        self.main_box.set_visible_child_name("sidebar")
+        self.main_box.bind_property("visible-child-name", titlebar, "visible-child-name", 
GObject.BindingFlags.SYNC_CREATE)
+
         start_pane_size_group = Gtk.SizeGroup(Gtk.SizeGroupMode.HORIZONTAL)
         start_pane_size_group.add_widget(left_box)
         start_pane_size_group.add_widget(self._left_header)