[pitivi] mediafilespreviewer: Clear the preview when no file is selected

[Alexandru Băluț <alexbalut src gnome org>]

commit b2bbe6eef6d1e6d0fa5471d60004c62f936b3146
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Mon Oct 3 12:13:06 2016 +0200

    mediafilespreviewer: Clear the preview when no file is selected
    
    Reviewed-by: Thibault Saunier <tsaunier gnome org>
    Differential Revision: https://phabricator.freedesktop.org/D1342

 pitivi/mediafilespreviewer.py |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/mediafilespreviewer.py b/pitivi/mediafilespreviewer.py
index 3c38307..6a94433 100644
--- a/pitivi/mediafilespreviewer.py
+++ b/pitivi/mediafilespreviewer.py
@@ -178,7 +178,9 @@ class PreviewWidget(Gtk.Grid, Loggable):
             file_chooser (Gtk.FileChooser): The file chooser providing the URI.
         """
         uri = file_chooser.get_preview_uri()
-        if uri is None or not uri_is_valid(uri):
+        previewable = uri and uri_is_valid(uri)
+        if not previewable:
+            self.clear_preview()
             return
         self.preview_uri(uri)