Customer order param is not working in notification templates.

[yecli]

Steps:

1. Open platform manager, Notifications tab;
2. Find Order paid notification, create it if it does not exist;
3. Select template, press Send;
4. Enter valid sender and recipient;
5. Enter something (e.g. order number) into the customer order field;
6. Press Send

Actual result:
500 error

Expected result:
No error. We should either have the ability to enter the data and process with it, or not see this field at all (or see it disabled).

Details:
Now this field is used as CustomerOrder object, and that causes the error - as it is just a string, in fact. We could mark such properties as "not manually set" by attribute (as it is set in the code), or not show it at all based on "complex" type.
Or we could have an ability to pass test object, e.g. using Json and restore full object on the server side.