hook: check for null object before introspection

vars/hook.groovy

@@ -6,6 +6,11 @@ def call(String name) {
         return
     }
     def hook = load(filepath)
+
+    if (hook == null) {
+        error("Hook '${filepath}' seems to be null. Did you forget to add 'return this' at the end of the script ?")
+    }
+
     if (hook.metaClass.respondsTo(hook, 'exec')) {
         hook.exec()
     } else {