Please post the actual stack trace and not a truncated screenshot of it, and when reporting please include what version of the add-on it is with.
Do you have Conversation Improvements installed? Or did you uninstall it at some point.
Can you run this SQL query? I need to see the search types list for your site.
SQL:
select content_type,field_value,addon_id
from xf_content_type_field
where field_name = 'search_handler_class' and content_type in ('conversation','conversation_message');