lib/Tools/electron

--- a/lib/Tools/electron	Tue Mar 15 13:22:37 2022 +0100
+++ b/lib/Tools/electron	Tue Mar 15 14:03:56 2022 +0100
@@ -4,4 +4,9 @@
 #
 # DESCRIPTION: run the Electron framework within the Isabelle environment
 
-exec "$ISABELLE_VSCODIUM_ELECTRON" "$@"
+if [ -z "$ISABELLE_VSCODIUM_ELECTRON" ]; then
+  echo '*** Undefined $ISABELLE_VSCODIUM_ELECTRON: missing "vscodium" component'
+  exit 2
+else
+  exec "$ISABELLE_VSCODIUM_ELECTRON" "$@"
+fi