null values supported, but not rendered properly yet.

jsonform/gadget_json_generated_form_child.js

@@ -16,6 +16,9 @@
   }
 
   function getDocumentType(doc) {
+    if (doc === null) {
+      return "null";
+    }
     if (doc instanceof Array) {
       return "array";
     }
@@ -729,7 +732,7 @@
       input = render_boolean(json_field, default_value);
     }
 
-    if (!input && ["string", "integer", "number"].indexOf(type) >= 0) {
+    if (!input && ["string", "integer", "number", "null"].indexOf(type) >= 0) {
       if (json_field.contentMediaType === "text/plain") {
         input = render_textarea(default_value, "string");
       } else {