[Composite.Forms.FormBuilder] Edit form field containing pre-defined field params in FormBuilder.config will freeze editor

[VolodymyrMuzyka]

How to reproduce:

1. Lets use standard field below (Notice it's have predefined field MaxSize):

  <Field name="multipleimages" label="${Composite.Forms.FormBuilder,Fields.MultipleImageUpload}" icon="icon-multipleimageupload" descriptionParameters="ButtonText,Help" validationParameters="Required,MaxFileCount" advancedParameters="Name">
    <f:function name="Composite.Forms.FormBuilder.Fields.MultipleFileUpload">
      <f:param name="MaxSize" value="20971520" />
      <f:param name="Accept" value="image/*" />
    </f:function>
  </Field>

2. Add MaxSize to markup:

                <Field name="multipleimages">
                    <f:param name="Label" value="Multiple Image Upload" />
                    <f:param name="Name" value="Multiple Image Upload" />
                    <f:param name="ButtonText" value="" />
                    <f:param name="Help" value="" />
                    <f:param name="Required" value="False" />
                    <f:param name="MaxFileCount" value="5" />
                    <f:param name="MaxSize" value="20971520" />
                </Field>

3. Go to visual mode, click on field, now try click to other - you will get frozen UI, in web browser console you will notice:
   Failed to load resource: the server responded with a status of 500 (Internal Server Error)

/Composite/InstalledPackages/content/views/Composite.Forms.FormBuilder/FieldEditor.aspx?consoleId=key6727764121954207

Expected: <f:param name="MaxFileCount" value="5" /> supposed to define default value and let editor edit this param in visual mode.