more robust: avoid crash of browser right-click menu;

--- a/src/Tools/jEdit/src/isabelle_session.scala	Wed Jan 30 22:39:58 2019 +0100
+++ b/src/Tools/jEdit/src/isabelle_session.scala	Wed Jan 30 22:46:49 2019 +0100
@@ -30,7 +30,7 @@
   val vfs_prefix = "isabelle-session:"
 
   class Session_Entry(name: String, path: String, marker: String)
-    extends VFSFile(name, path, null, VFSFile.FILE, 0L, false)
+    extends VFSFile(name, path, vfs_prefix + name, VFSFile.FILE, 0L, false)
   {
     override def getPathMarker: String = marker