Bug
It happens in specific conditions, regardless of Agile Dashboard activation. I could reproduce it with AgileDashboard service disabled.
Plugin AgileDashboard can add reports search criteria. It adds an "In Milestone" criterion to sort artifacts by their milestone. To achieve this, plugin Tracker sends the TRACKER_EVENT_REPORT_DISPLAY_ADDITIONAL_CRITERIA event, and plugin AgileDashboard always responds.
The bug only happens when the user has no permission access on any of the milestone trackers.
For example, given the tracker hierarchy Releases -> Sprints and given that in my scrum configuration I have two plannings: Release Planning where I can prioritize User stories and Sprint Planning where I can also prioritize User stories AND given only members of the group user_group can view Releases and Sprints, when as user Alice (NOT member of user_group) I try to access the tracker User stories' page then there will be a PHP fatal error.
In my example, if Alice has access to either Releases or Sprints then there is no error and she can view the page.
Known workaround
Grant Alice access to at least one milestone tracker (e.g. Releases) by adding her to user_group or by granting read permissions on Releases for registered_users