I can send mesg to customer like tracking detail about goods

Shown this error today when i click on button "Add History", i was trying to send shipping detail, trackning number etc to customer

SyntaxError: Unexpected token <
OK
<b>Notice</b>: Undefined variable: json in <b>/customers/8/e/3/xxxxxxxxx.se/httpd.www/admin/controller/sale/order.php</b> on line <b>2221</b>

what should i do to fix this?