admin: fix/avoid error thrown when fieldType/widget doesn't exist

there might be case from manual editing or other (e.g when you have a schema where `ui:widget` doesnt exist in the formule context widgets), that it will throw an error and block fnctionality for the whole app

We can try and handle better

example where it breaks: https://github.com/cern-sis/react-formule/blob/master/src/admin/components/PropKeyEditorForm.jsx#L60

we can avoid it , with other workarounds

Start in src/admin/components/PropKeyEditorForm.jsx around line 60 and reproduce the failure with a schema whose ui:widget is absent from the formule context widgets. Determine how the admin should handle the missing fieldType or widget, and confirm that it no longer throws an error or blocks the application.

Written by the indexing model from the issue text.