Reproduction case:
1. In a public project add a kanban widget in the one of the project's dashboard
2. Restrict the permission of the tracker used by the kanban so an anonymous user can not see it
3. Try to access the dashboard with the kanban widget

This issue has been reported by @wicope on the community chat.

Trace:
PHP message: PHP Fatal error: Uncaught exception 'AgileDashboard_KanbanCannotAccessException' in /usr/share/tuleap/plugins/agiledashboard/include/AgileDashboard/KanbanFactory.class.php:65
Stack trace:
#0 /usr/share/tuleap/plugins/agiledashboard/include/AgileDashboard/Widget/Kanban.php(114): AgileDashboard_KanbanFactory->getKanban(Object(PFUser), '14')
#1 /usr/share/tuleap/src/common/Dashboard/Widget/DashboardWidgetPresenterBuilder.php(97): Tuleap\AgileDashboard\Widget\Kanban->loadContent('2')
#2 /usr/share/tuleap/src/common/Dashboard/Widget/DashboardWidgetPresenterBuilder.php(76): Tuleap\Dashboard\Widget\DashboardWidgetPresenterBuilder->getWidgetsPresenterByColumn(Object(Tuleap\Dashboard\Project\ProjectDashboard), Object(Tuleap\Dashboard\Widget\OwnerInfo), Object(Tuleap\Dashboard\Widget\DashboardWidgetColumn), false)
#3 /usr/share/tuleap/src/common/Dashboard/Widget/DashboardWidgetPresenterBuilder.php(54): Tuleap\Dashboard\Widget\DashboardWidgetPresenterBuilder->getColumnsPresenterByLine(Object(Tuleap\Das" while reading response header from upstream, client: 172.18.0.1, server: tuleap-web.tuleap-aio-dev.docker, request: "GET /projects/project1/?dashboard_id=56 HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "tuleap-web.tuleap-aio-dev.docker"