slightly more robust Plugin.stop -- components might refer to Isabelle.system even after shutdown;

--- a/src/Tools/jEdit/src/jedit/plugin.scala	Mon Sep 06 22:58:06 2010 +0200
+++ b/src/Tools/jEdit/src/jedit/plugin.scala	Tue Sep 07 13:16:45 2010 +0200
@@ -286,10 +286,9 @@
     Isabelle.setup_tooltips()
   }
 
-  override def stop()
+  override def stop()  // FIXME fragile
   {
     Isabelle.session.stop()  // FIXME dialog!?
     Isabelle.session = null
-    Isabelle.system = null
   }
 }