Typing while previewing an item that wasn't opened via a search led to:
  Gtk-CRITICAL **: gtk_widget_realize: assertion
    'widget->priv->anchored || GTK_IS_INVISIBLE (widget)' failed
  Gtk-CRITICAL **: gtk_widget_event: assertion
    'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed

This code is different from gnome-documents. It uses different
MainToolbar sub-classes for each mode, while here the same MainToolbar
instance is shared.

The check to ensure that the Searchbar has a parent needs to be
restored to avoid these CRITICALs.

Fallout from 9bdf053a

https://bugzilla.gnome.org/show_bug.cgi?id=776082