Describe the bug
The docblock of EventTimelineSets constructor states that the room should be nullable, the parameters however state that it is not. Also the usage within the class is not clear how it should be used. (getPendingEvents has a guard clause, while aggregateRelations does not)

This leads to errors if you use the Timelineset without providing a room.

To Reproduce
Steps to reproduce the behavior:

1. Create a new EventTimelineSet with room set to null
2. Try to add an Event with a relation (in our case m.replace)

Expected behavior
The class should work correctly without a room or the docblock should be changed.

Desktop (please complete the following information):

• OS: Ubuntu
• Browser: chrome, firefox