Put pr # a (without the space) in a field transforming the cross references (e.g. a tracker text field) to reproduce the crash.

Trace:

PHP Fatal error:  Uncaught TypeError: Tuleap\PullRequest\PullRequestRetriever::getPullRequestById(): Argument #1 ($id) must be of type int, string given, called in /usr/share/tuleap/plugins/pullrequest/include/PullRequest/Reference/ReferenceFactory.php on line 39 and defined in /usr/share/tuleap/plugins/pullrequest/include/PullRequest/PullRequestRetriever.php:39
Stack trace:
#0 /usr/share/tuleap/plugins/pullrequest/include/PullRequest/Reference/ReferenceFactory.php(39): Tuleap\PullRequest\PullRequestRetriever->getPullRequestById()
#1 /usr/share/tuleap/plugins/pullrequest/include/pullrequestPlugin.php(372): Tuleap\PullRequest\Reference\ReferenceFactory->getReferenceByPullRequestId()
#2 /usr/share/tuleap/src/common/Event/EventManager.class.php(128): pullrequestPlugin->getReference()
#3 /usr/share/tuleap/src/common/Plugin/PluginLoader.php(73): EventManager->eventManagerDispatch()