Activity stream does not load if relationship changes are in the feed and modifications were made by an invalid user or a user that no longer exists.
Steps to Reproduce
Impersonate an admin user, for example, Fred Luddy from the demo data.
Navigate to Incident > Open.
Select any Incident and note its number.
Right-click in the form header and choose Configure > Related Lists.
Add Task->Parent to the Selected list and click Save.
Go to the Tasks related list on your Incident and click Edit.
Move any Task to selected and click Save.
An activity stream entry for your relationship change is displayed.
Delete or change the user ID from the admin account (in this example, Fred Luddy).
Reload the Incident from earlier.
Note that instead of the user ID for the invalid account being displayed, the activity stream doesn't load at all and NPE errors are listed in the logs.
Create an inactive user with a matching user ID of the user who was deleted or is otherwise no longer valid.
If you are able to upgrade, review the Fixed In field to note the versions that have a permanent fix.
Related Problem: PRB720998