Unable to turn OFF 'Allow unassigned issues' in General configuration,

Cannot turn off "Allow unassigned issues" as there are 31 issue(s) that are unassigned.
Please assign these issues before disabling Unassigned Issues.
N.B. You may not have permissions to view all of these issues 

Problem : field 'assignee' has been hide on certain screen

Workaround :
1. Go to Jira Administrator > Field configurations > Decision Configuration & News Configuration > Configure, show the assignee field.
2. Re-index Jira
3. Then you should be able to view the unassigned issues in General configuration
4. Bulk edit to assign those issues
5. Turn OFF 'Allow Unassigned Issues' in General configuration

Suggestion : to have better handling on such case instead of spending time to fill in virtual assignee to the issues.