Error: Class name must be a valid object or a string

I reviewed this patch it's solving the error but on this page i am having this warning
Undefined property: Drupal\solr_qb\Plugin\SolrQbDriver\CustomDriver::$PluginBase in Drupal\solr_qb\Form\SolrQbSettingsForm->Drupal\solr_qb\Form\{closure}() (line 148 of modules\contrib\solr_qb\src\Form\SolrQbSettingsForm.php).

Hi Dheeraj, its working for me.
Please clear cache and try

Reviewed on this issue and reproduced. After applying the patch error got resolved successfully. Attached before and after screenshots. Good to move RTBC..

@kushagra.goyal can you mark as RTBC?

I have review the issue and the MR !2 fixed the issue.
Marking this ticket as RTBC!! and thank you

Thanks all,