Plugin.stop: refrain from invalidating Isabelle.session -- some actors/dockables out there might still refer to it;

--- a/src/Tools/jEdit/src/jedit/plugin.scala	Thu Sep 23 15:36:03 2010 +0200
+++ b/src/Tools/jEdit/src/jedit/plugin.scala	Thu Sep 23 16:48:12 2010 +0200
@@ -320,14 +320,12 @@
   {
     Isabelle.system = new Isabelle_System
     Isabelle.system.install_fonts()
-    Isabelle.session = new Session(Isabelle.system)  // FIXME dialog!?
-
+    Isabelle.session = new Session(Isabelle.system)
     Isabelle.setup_tooltips()
   }
 
-  override def stop()  // FIXME fragile
+  override def stop()
   {
-    Isabelle.session.stop()  // FIXME dialog!?
-    Isabelle.session = null
+    Isabelle.session.stop()
   }
 }