When a user is null in track-changes UI, display them as "Anonymous".

services/web/app/views/project/editor/track-changes.jade

@@ -74,9 +74,11 @@ div#trackChanges(ng-show="ui.view == 'track-changes'")
 							span.doc {{ doc.entity.name }}
 						div.users
 							div.user(ng-repeat="update_user in update.meta.users")
-								.color-square(ng-style="{'background-color': 'hsl({{ update_user.hue }}, 70%, 50%)'}")
+								.color-square(ng-if="update_user != null", ng-style="{'background-color': 'hsl({{ update_user.hue }}, 70%, 50%)'}")
+								.color-square(ng-if="update_user == null", ng-style="{'background-color': 'hsl(100, 70%, 50%)'}")
 								.name(ng-if="update_user.id != user.id") {{update_user.first_name}} {{update_user.last_name}}
 								.name(ng-if="update_user.id == user.id") You
+								.name(ng-if="update_user == null") #{translate("anonymous")}
 							div.user(ng-if="update.meta.users.length == 0")
 								.color-square(style="background-color: hsl(100, 100%, 50%)")
 								span #{translate("anonymous")}

services/web/public/coffee/ide/track-changes/TrackChangesManager.coffee

@@ -187,7 +187,8 @@ define [
 					doc.entity = @ide.fileTreeManager.findEntityById(doc_id, includeDeleted: true)
 
 				for user in update.meta.users or []
-					user.hue = @ide.onlineUsersManager.getHueForUserId(user.id)
+					if user?
+						user.hue = @ide.onlineUsersManager.getHueForUserId(user.id)
 
 				if !previousUpdate? or !moment(previousUpdate.meta.end_ts).isSame(update.meta.end_ts, "day")
 					update.meta.first_in_day = true