Removing a dashboard owner/viewer from a group the dashboard/filter has been shared results in the owner being unable to edit the dashboard/filter

Issue Summary

Removing a dashboard or filter owner/viewer from a group the dashboard has been shared results in the owner being unable to edit the dashboard.
It also happens if you have "Allow users to share dashboards and filters with the public." enabled (in General Configuration) and share the Dashboard with the Public and then unable the setting back.

Steps to Reproduce

1. Create a dashboard/filter
2. Share the dashboard with a specific group that you are one of there member.
3. Login as Administrator, remove the user that shared the dashboard from the group they share it with.

Expected Results

The owner will be able to see their dashboard and edit it/add and remove gadgets.

Actual Results

The owner will be able to see their dashboard/filter but can't edit it. This seems to be due to a validation that requires the user to be part of a group to be able to share the dashboard/filter with that group.

Workaround

Make sure the owner is included in all the groups the dashboard/filter is shared with.