posting now so I can add details later if needed. Happened after I enabled flow on my talk page.
( ! ) Fatal error: Call to a member function toString() on a non-object in .../includes/mail/UserMailer.php on line 203
Call Stack
# Time Memory Function Location
1 0.0004 243168 {main}( ) .../index.php:0
2 0.6374 29745200 MediaWiki->run( ) .../index.php:41
3 1.7639 45404312 MediaWiki->restInPeace( ) .../MediaWiki.php:426
4 1.7639 45404536 DeferredUpdates::doUpdates( ) .../MediaWiki.php:589
5 2.3453 53880328 MWCallableUpdate->doUpdate( ) .../DeferredUpdates.php:105
6 2.3453 53880376 call_user_func:{.../includes/deferred/CallableUpdate.php:27} ( ) .../CallableUpdate.php:27
7 2.3453 53880376 EchoHooks::{closure:.../extensions/Echo/Hooks.php:404-406}( ) .../CallableUpdate.php:27
8 2.3472 54041192 EchoDiscussionParser::generateEventsForRevision( ) .../Hooks.php:405
9 2.3642 54240592 EchoEvent::create( ) .../DiscussionParser.php:70
10 2.3667 54272320 EchoNotificationController::notify( ) .../Event.php:150
11 2.3830 54491568 EchoNotificationController::doNotification( ) .../NotificationController.php:100
12 2.3830 54492008 call_user_func_array:{.../extensions/Echo/includes/controller/NotificationController.php:262} ( ) .../NotificationController.php:262