Importing theme colours from Slack gives an error

Description

5.18, 5.19, master

Repro steps:
1. Attempt to import a Slack Theme from Account Settings > Display > Theme > Import Theme Colours from Slack.

Observed:
See error in screenshot.

QA Test Steps

None

Reporter

None

Status

Mana

None

Assignee

Joram Wilander

QA Assignee

Steve Mudie

Reporter

Amy Blais

Epic Link

None

Fix versions

Mattermost Team

Sustained Engineering

Sprint

None

Labels

QA Testing Areas

None

GitHub Issue

None

Components

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