R1:29df80b48f16

Fix a fatal during breadcrumb construction when viewing a dashboard you don't have permission to view

Summary: Ref PHI662. Viewing a dashboard you don't have permission to view (in the Dashboard application) currently fatals while building crumbs, since we fail to build the ` ... > Dashboard 123 > ...` crumb.

Test Plan:
- Viewed a dashboard I didn't have permission to view in the Dashboards application.
- Before patch, fatal when calling `getID()` on a non-object.
- After patch, sensible policy…