EXCEPTION: (RuntimeException) Undefined index:  at [<arcanist>/src/error/PhutilErrorHandler.php:251]
 arcanist(), ava(), phorge(), translations(), wmf-ext-misc()
   #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phorge>/src/applications/differential/engine/DifferentialRevisionTimelineEngine.php:24]
   #1 <#2> DifferentialRevisionTimelineEngine::newTimelineView() called at [<phorge>/src/applications/transactions/engine/PhabricatorTimelineEngine.php:70]
   #2 <#2> PhabricatorTimelineEngine::buildTimelineView() called at [<phorge>/src/applications/base/controller/PhabricatorController.php:524]
   #3 <#2> PhabricatorController::buildTransactionTimeline(DifferentialRevision, DifferentialTransactionQuery, NULL, array) called at [<phorge>/src/applications/transactions/controller/PhabricatorApplicationTransactionShowOlderController.php:38]
   #4 <#2> PhabricatorApplicationTransactionShowOlderController::handleRequest(AphrontRequest) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:284]
   #5 phlog(RuntimeException) called at [<phorge>/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php:41]
   #6 PhabricatorDefaultRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:751]
   #7 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:296]
   #8 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:204]
   #9 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phorge>/webroot/index.php:35]

$view_data = $this->getViewData();
if (!$view_data) {
  $view_data = array();
}

$left = idx($view_data, 'left');
$right = idx($view_data, 'right');

$diffs = id(new DifferentialDiffQuery())
  ->setViewer($viewer)
  ->withIDs(array($left, $right))
  ->execute();
$diffs = mpull($diffs, null, 'getID');
$left_diff = $diffs[$left];     // <- line that throws the exception
$right_diff = $diffs[$right];