Reporter system field throws ClassCastException after populateFromIssue() and validateFromParams()

When you populate the reporter system field from the issue using ReporterSystemField#populateFromIssue() then validating the field value with ReporterSystemField#validateParams() you get a ClassCastException.

Steps to reproduce:

1. populate reporter field with populateFromIssue() stores User into the field values holder
2. validate the reporter field with validateParams() which is expecting a String and casts the field value to which causes the ClassCastException