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

[pamfilos]

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