Fix Monochrome Setting Crash

[thecoolwinter]

Description

Fixes a crash with the monochrome setting. The icon color value's willSet was being called before outlineView was set up, causing an unexpected nil exception. This adds an optional to the call so it's safe to call now.

Related Issues

• closes 🐞 Changing folders to monochrome causes app to crash on restart #1871

Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

N/A