--- a/FlyByWindowInformation.st	Mon Feb 07 12:26:34 2011 +0100
+++ b/FlyByWindowInformation.st	Mon Feb 28 19:08:00 2011 +0100
@@ -97,7 +97,11 @@
 
     key == $? ifTrue:[
         [
-            WindowTreeView openOn:(lastView topView) initialSelection:lastView.
+            WindowTreeView notNil ifTrue:[
+                WindowTreeView openOn:(lastView topView) initialSelection:lastView.
+            ] ifFalse:[
+                self warn:'WindowTreeView class is not present!!'.
+            ].
         ] fork.
         self stop.
         ^ true.