Update multiple.js (#21)

Author: Kanabanarama

src/editors/multiple.js

@@ -87,7 +87,7 @@ JSONEditor.defaults.editors.multiple = JSONEditor.AbstractEditor.extend({
       schema = self.jsoneditor.expandRefs(schema);
 
       // If we need to merge `required` arrays
-      if(type.required && Array.isArray(type.required) && self.schema.required && Array.isArray(self.schema.required)) {
+      if(type && type.required && Array.isArray(type.required) && self.schema.required && Array.isArray(self.schema.required)) {
         schema.required = self.schema.required.concat(type.required);
       }
     }