WSOD on sublanding page / topic list builder if referencing a deleted topic

[andybroomfield]

Seeing this error, which prevents a page load

Error: Call to a member function label() on null in localgov_services_sublanding_preprocess_paragraph__topic_list_builder() (line 132 of /var/www/html/docroot/modules/contrib/localgov_services/modules/localgov_services_sublanding/localgov_services_sublanding.module).

This happens becuase on an older page, if a topic is deleted and there is no custom header value, then you get a WSOD.

Steps to reproduce:

1. Create a topic
2. Create a sub landing page, using the topic list builder, set the topic as the one previously created, leave the header blank and save.
3. Delete the referenced topic
4. Flush caches and view the sublanding page.

Expected result:
Sub landing page displays, allbeit the heading is no longer present.

Actual result:
WSOD and above error in logs.