[REF] Ensure that the form param _id is set when adding a new group w…

[seamuslee001]

…ith the newly created group id so that consumers of the hook_civicrm_postProcess can access the id

Overview

This ensures that the form parameter $this->_id is set to the newly created id / the id of the current group so that hook_civicrm_postProcess consumers can use it in their code

Before

_id parameter not always set

After

_id parameter always set when we get to postProcess

ping @eileenmcnaughton @Edzelopez @mattwire

[civibot]

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[eileenmcnaughton]

Well there is precedent for this on other forms & the reason is commented so I guess it's fine