From production logs in the cluster:
[2020-04-28 10:12:17] EXCEPTION: (InvalidArgumentException) Argument 1 passed to HarbormasterBuildPlanBehavior::getPlanOption() must be an instance of HarbormasterBuildPlan, null given, called in /core/lib/phabricator/src/applications/harbormaster/storage/build/HarbormasterBuild.php on line 239 and defined at [<arcanist>/src/error/PhutilErrorHandler.php:227] arcanist(head=stable, ref.master=7d15b85a1bc0, ref.stable=5efbe22c2afa), libcore(), phabricator(head=stable, ref.stable=e17c49699309), services(head=stable, ref.master=fb2140e5b9da, ref.stable=2d7586076ae4) #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phabricator>/src/applications/harbormaster/plan/HarbormasterBuildPlanBehavior.php:122] #1 <#2> HarbormasterBuildPlanBehavior::getPlanOption(NULL) called at [<phabricator>/src/applications/harbormaster/storage/build/HarbormasterBuild.php:239] #2 <#2> HarbormasterBuild::assertCanRestartBuild() called at [<phabricator>/src/applications/harbormaster/storage/build/HarbormasterBuild.php:219] #3 <#2> HarbormasterBuild::canRestartBuild() called at [<phabricator>/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php:233] #4 <#2> HarbormasterBuildableViewController::buildBuildList(HarbormasterBuildable) called at [<phabricator>/src/applications/harbormaster/controller/HarbormasterBuildableViewController.php:35] #5 <#2> HarbormasterBuildableViewController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:288] #6 phlog(InvalidArgumentException) called at [<phabricator>/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php:41] #7 PhabricatorDefaultRequestExceptionHandler::handleRequestThrowable(AphrontRequest, InvalidArgumentException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:752] #8 AphrontApplicationConfiguration::handleThrowable(InvalidArgumentException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:300] #9 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:211] #10 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:35]