more robust init;

--- a/src/Pure/System/isabelle_system.scala	Sat Nov 11 20:01:14 2023 +0100
+++ b/src/Pure/System/isabelle_system.scala	Sat Nov 11 20:08:20 2023 +0100
@@ -74,6 +74,7 @@
     synchronized {
       if (_classpath.isEmpty) _classpath = Some(Classpath())
     }
+    Registry.global
   }