Error ([500]) in channels imported from Slack with Full Name option enabled

Description:

Error ([500]) in channels imported from Slack with Full Name option enabled

Steps to reproduce:

1. Import Channels and users from slack
2. Enable "Use Real Name" option on Layout -> User interface
3. Click on a Channel imported from slack with lots of messages
4. Error [500] is present

Expected behavior:

No error in opening a channel imported from Slack

Actual behavior:

Server Setup Information:

• Version of Rocket.Chat Server: 3.6.1
• Operating System: Linux
• Deployment Method: docker
• Number of Running Instances: 1
• DB Replicaset Oplog:
• NodeJS Version: v12.16.1
• MongoDB Version: 4.0.20

Client Setup Information

• Desktop App or Browser Version: Google Chrome 85.0.4183.102
• Operating System: Windows

Additional context

Enable "Use Real Name" option on Layout -> User interface

Relevant logs:

I20200915-18:48:07.978(0) Exception while invoking method 'loadHistory' TypeError: Cannot destructure property 'username' of '_ref' as it is null. at app/utils/server/lib/normalizeMessagesForUser.js:34:39 at Array.forEach () at app/utils/server/lib/normalizeMessagesForUser.js:34:28 at Array.forEach () at normalizeMessagesForUser (app/utils/server/lib/normalizeMessagesForUser.js:26:11) at loadMessageHistory (app/lib/server/functions/loadMessageHistory.js:31:19) at MethodInvocation.loadHistory (server/methods/loadHistory.js:33:10) at MethodInvocation.methodsMap. (app/lib/server/lib/debug.js:67:34) at maybeAuditArgumentChecks (packages/ddp-server/livedata_server.js:1771:12) at packages/ddp-server/livedata_server.js:719:19 at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js🔢12) at packages/ddp-server/livedata_server.js:717:46 at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js🔢12) at packages/ddp-server/livedata_server.js:715:46 at new Promise () at Session.method (packages/ddp-server/livedata_server.js:689:23) at packages/ddp-server/livedata_server.js:559:43

Start with app/utils/server/lib/normalizeMessagesForUser.js at line 34, then trace its caller in app/lib/server/functions/loadMessageHistory.js and server/methods/loadHistory.js. Reproduce the Slack channel import with Use Real Name enabled and confirm that opening a channel with many messages no longer returns a 500 error.