Debuging Solr Search in Silverstripe

Question

I have a silverstripe site that uses solr for search. It has developed an error:

PHP Warning:  Attempt to read property "ClassName" on null in /var/www/stage/public_html/vendor/firesphere/solr-search/src/Services/SolrCoreService.php on line 174
ERROR [Warning]: Attempt to read property "ClassName" on null
IN GET dev/tasks/SolrClearDirtyClasses
Line 174 in /var/www/stage/public_html/vendor/firesphere/solr-search/src/Services/SolrCoreService.php

Config:

MySearch.yml
---
Name: MySolrSearch
After: SolrSearch

---
Firesphere\SolrSearch\Services\SolrCoreService:
  config:
    endpoint:
      localhost:
        host: localhost
        port: 8983
        timeout: 60
        username: solruser
        password: xxxxxxxx
        # set up timeouts
        index_timeout: 90
        optimize_timeout: 200
        finalize_timeout: 600
        http_method: 'AUTO'
        # commit within 60ms
        commit_within: 120
  store:
    mode: 'file'
    path: '/var/solr/data'
  cpucores: 2

Modules:

Silverstripe: 4.10
silverstripe-solr-search : dev/master

What steps can I take to debug this?