Missing entry in fieldconfigschemeissuetype results in errors when trying to add issues types to issue scheme

Issue Summary

Missing entry in fieldconfigschemeissuetype causes HTTP 500 Internal Server Error when linking issue types to issue type scheme on the UI.

Identify and address the root cause for the record deletion in the table.

Steps to Reproduce

We don't know the root cause or actions that lead to this database inconsistency. 

Actual Results

500 internal server error screen presented to the user. The below exception is thrown in logs :

There is not exactly one config for this scheme (<field-scheme-id>). Configs are {}. 

Or

issue type schemes should have exactly one field config

Expected Results

No database inconsistencies and should be able to link  issue types to issue type scheme on the UI.

Workaround

Identify the missing entry and insert an entry manually:

insert into fieldconfigschemeissuetype (fieldconfigscheme,fieldconfiguration) values (<fieldconfigscheme-id>, <fieldconfiguration-id>) returning *