keep PIDE.plugin for the sake of still open dockables etc. -- jEdit exits these *after* the stop operation;

--- a/src/Tools/jEdit/src/plugin.scala	Wed Mar 15 16:55:37 2017 +0100
+++ b/src/Tools/jEdit/src/plugin.scala	Wed Mar 15 16:58:52 2017 +0100
@@ -458,7 +458,5 @@
     exit_models(JEdit_Lib.jedit_buffers().toList)
     session.stop()
     file_watcher.shutdown()
-
-    PIDE._plugin = null
   }
 }