An error when try to merge ticket (ticket drop down list) website_support V11.0.1.6.1

[MohsenWaleed]

I got the below error once try to select the ticket to merge:

Error:
Uncaught TypeError: value.split is not a function

http://localhost:8069/web/static/src/js/fields/relational_fields.js:273
Traceback:
TypeError: value.split is not a function
at Class.getDisplayName (http://localhost:8069/web/static/src/js/fields/relational_fields.js:273:22)
at http://localhost:8069/web/static/src/js/fields/relational_fields.js:393:33
at Function..map._.collect (http://localhost:8069/web/static/lib/underscore/underscore.js:164:24)
at Object. (http://localhost:8069/web/static/src/js/fields/relational_fields.js:392:32)
at Object. (http://localhost:8069/web/static/lib/jquery/jquery.js:3276:89)
at fire (http://localhost:8069/web/static/lib/jquery/jquery.js:3119:58)
at Object.fireWith [as resolveWith] (http://localhost:8069/web/static/lib/jquery/jquery.js:3231:49)
at Object.deferred.(anonymous function) [as resolve] (http://localhost:8069/web/static/lib/jquery/jquery.js:3321:62)
at Object. (http://localhost:8069/web/static/src/js/services/ajax_service.js:13:33)
at Object. (http://localhost:8069/web/static/lib/jquery/jquery.js:3276:89)

I discovered that the subject field of the ticket is empty so need to make it Required.