[kupfer] windows: Valid check for windows

[Ulrik Sverdrup <usverdrup src gnome org>]

commit da3bbe40c268ed2de453f1ad640392577c1c4247
Author: Ulrik Sverdrup <ulrik sverdrup gmail com>
Date:   Fri Jan 8 01:45:50 2010 +0100

    windows: Valid check for windows

 kupfer/plugin/windows.py |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/kupfer/plugin/windows.py b/kupfer/plugin/windows.py
index d90de66..955571f 100644
--- a/kupfer/plugin/windows.py
+++ b/kupfer/plugin/windows.py
@@ -40,6 +40,9 @@ class WindowLeaf (Leaf):
 				icon="gtk-add")
 		yield WindowAction(_("Close"), "close", time=True, icon="gtk-close")
 
+	def is_valid(self):
+		return self.object and self.object.get_xid()
+
 	def get_description(self):
 		workspace = self.object.get_workspace()
 		nr, name = workspace.get_number(), workspace.get_name()