When I go to a Conpherence room, there will sometimes pop up following modal:
{F313618}
Stack trace:
```
arcanist(head=arcpatch-D25374, ref.master=df6c315ace5f, ref.arcpatch-D25374=120a37eb278d), phorge(head=master, ref.master=0ef08baf6452)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<phorge>/src/applications/conpherence/storage/ConpherenceThread.php:91]
#1 <#2> ConpherenceThread::getParticipant(string) called at [<phorge>/src/applications/conpherence/controller/ConpherenceUpdateController.php:331]
#2 <#2> ConpherenceUpdateController::loadAndRenderUpdates(string, string, integer) called at [<phorge>/src/applications/conpherence/controller/ConpherenceUpdateController.php:147]
#3 <#2> ConpherenceUpdateController::handleRequest(AphrontRequest) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:284]
#4 phlog(RuntimeException) called at [<phorge>/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php:41]
#5 PhabricatorDefaultRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:751]
#6 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:296]
#7 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phorge>/src/aphront/configuration/AphrontApplicationConfiguration.php:204]
#8 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phorge>/webroot/index.php:35]
```
The phid is my user phid.
== Steps to reproduce ==
* Have Aphlict enabled and working
* Be user Alice and visit a Conference Room __without__ Alice in participants list
* Be user Bob and send a message in that Conference Room
→ Crash from the tab of Alice
This is fixed when Alice joins the room or she sends a message (since it auto-joins the room).