R1:9e54e6e88661

Fix an undefined variable

Summary:
The `$timeline` variable is undefined. I was seeing the following error in the logs:

```
EXCEPTION: (RuntimeException) Undefined variable: timeline at [<phutil>/src/error/PhutilErrorHandler.php:210]
#0 PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/harbormaster/controller/HarbormasterStepEditController.php:205]
#1 HarbormasterStepEditController::processRequest() called at…