Exception if entity reference is not valid (got deleted)

Problem/Motivation

If the entity reference used for the link for some reason (entity was deleted) does no longer exist, field group link creates an exception:

Call to a member function toUrl() on null in Drupal\field_group_link\Plugin\field_group\FieldGroupFormatter\Link->getUrlFromField()

While it is probably bad habit to delete entities that are referenced elsewhere, this is something that can (easily) happen and should not result in WSOD imho.

Steps to reproduce

• Use a reference field for the link
• delete the entity that is referenced in the field
• visit the page where the link should be shown

Proposed resolution

Check if there is a valid entity reference (like there is already a check for a valid file entity if file is used) and skip the url generation if there is no valid entity reference

Remaining tasks

User interface changes

na

API changes

na

Data model changes

na