Break can occur when viewing manage members list while another user adds a member

Description

A break can occur when you view the managed members list while another user adds members.

Steps:
1. In 2 separate Chrome sessions, login as different users viewing the same channel.
2. Ensure the channel does not already have a post from the user to be added to the channel.
3. As UserA, refresh the page (to clear any cache from viewing other channels), then open the manage members and leave the modal open.
3. As UserB open the manage members list and add a user.

Expected: *UserA * can see the newly added user
Actual: UserA page fails to display, as a result of a JavaScript exception

You may need to repeat step 3 a few times.

TypeError: Cannot read property 'delete_at' of undefined
See Screen Attached.

QA Test Steps

Steps as updated in issue are accurate capture steps.

Mana

None

Assignee

Jesse Hallam

QA Assignee

Linda Mitchell

Reporter

Dylan Haussermann

Epic Link

None

Fix versions

Mattermost Team

ABC

Sprint

None

Labels

None

QA Testing Areas

Channel Settings

GitHub Issue

None

Components

None

Severity

None
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