Don't register search_box if not on a page

issue2770
(transplanted from cda1bcf226dc18140e6fa6f9e884b9be2ea29c7a)

tryton/gui/window/view_form/view/screen_container.py

@@ -331,10 +331,10 @@ class ScreenContainer(object):
         from tryton.gui.main import Main
         page = Main.get_main().get_page()
         if button.get_active():
-            if self.search_window not in page.dialogs:
+            if page and self.search_window not in page.dialogs:
                 page.dialogs.append(self.search_window)
             self.search_window.show()
         else:
             self.search_window.hide()
-            if self.search_window in page.dialogs:
+            if page and self.search_window in page.dialogs:
                 page.dialogs.remove(self.search_window)