Fix crash in alias settings page (#18004)

app/models/account_alias.rb

@@ -28,6 +28,11 @@ class AccountAlias < ApplicationRecord
     super(val.start_with?('@') ? val[1..-1] : val)
   end
 
+  def pretty_acct
+    username, domain = acct.split('@')
+    domain.nil? ? username : "#{username}@#{Addressable::IDNA.to_unicode(domain)}"
+  end
+
   private
 
   def set_uri