Rendering of RichTextWidget fails if pattern_options contain functions

[marwoe]

I accidentally came across an error that occurs under certain circumstances when rendering the RichTextWidget. The error occurs under the following conditions: you are using a RichText field with pattern_options that contain a function and there are more than one allowed formats for the field - which is the default (text/html, text/x-web-textile) for new Plone Sites. If you try to edit such a form this leads to the following error:

  Module plone.app.z3cform.widgets.richtext, line 104, in render
  Module plone.app.z3cform.widgets.richtext, line 178, in tinymce_richtextwidget_render
  Module plone.app.z3cform.widgets.richtext, line 146, in render_input_mode
  Module json, line 231, in dumps
  Module json.encoder, line 199, in encode
  Module json.encoder, line 257, in iterencode
  Module json.encoder, line 179, in default
TypeError: Object of type function is not JSON serializable

The problem lies in the render_input_mode method. With only one allowed format the base render method is used, which runs the call_callables method on the pattern_options which resolves the function calls. With multiple allowed formats the call_callables call is missing and the functions (in the patterns) will not be resolved.

This happens with version 4.3.2 and latest Plone version (6.0.11.1, Classic UI) - but as far as i can see this should also happens with every newer version.