Steps to reproduce the issue

• Set custom_reply parameter to true in com_contact
• Try to send a message using API

Expected result

• No exception

Actual result

• Exception thrown

System information (as much as possible)

At least from 5.2 but I assume that's older issue.

Additional comments

When custom_reply is set, no message is sent so $sent is still at false.

So this check will result in exception even though the message was processed by a plugin without any issue: