more robust (again): allow to deactivate main plugin;

--- a/src/Tools/jEdit/src/main_plugin.scala	Thu Jul 15 21:11:39 2021 +0200
+++ b/src/Tools/jEdit/src/main_plugin.scala	Thu Jul 15 21:35:31 2021 +0200
@@ -495,7 +495,5 @@
     session.stop()
     file_watcher.shutdown()
     PIDE.editor.shutdown()
-
-    PIDE._plugin = null
   }
 }