Customized Form json data in flowable-modeler

Hi, when I build a form in flowable-modeler, I want to customize/add some properties to the form fields. I invade into the code of flowable-modeler, and add an extra property to form field as below:

this.saveForm = function (data, name, formKey, description, saveCallback, errorCallback) {
var fieldIndex = 0;
data.formRepresentation = $rootScope.currentForm; = name;
data.formRepresentation.key = formKey;

        if (description && description.length > 0) {
            data.formRepresentation.description = description;
        for (var i = 0; i < $rootScope.formItems.length; i++) {

$rootScope.formItems[i].testproperty=‘testproperty’; // Here I add an extra property to form field
var field = $rootScope.formItems[i];

            this._cleanFormField(field, fieldIndex++);

And then, the application cannot work well, the console gives the error:
angular.js:9827 PUT http://localhost:8080/flowable-modeler/app/rest/form-models/77e877c2-0099-11e8-a238-0a7508fb1bfa 400 (),
So it cannot save the form data to the database, does it mean that I cannot customize my form data and send it to the back-end?