take in account schema can be boolean in metaSchema detection function

jsonform/gadget_json_generated_form_child.js

@@ -831,6 +831,9 @@
   }
 
   function checkSchemaIsMetaSchema(schema) {
+    if (!schema) {
+      return false;
+    }
     if (schema instanceof Array) {
       var i,
         sch;