Fix the OmnibarLayout crash

[0nko]

Task/Issue URL: https://app.asana.com/0/488551667048375/1208618045189439/f

Description

We need to make sure all attempts to access the OmnibarLayout’s ViewModel are checked to make sure the window is attached, otherwise the view may not be injected yet.

Steps to test this PR

• Enable Don’t keep activities option in Developer options
• Clear the app storage and start the app
• Go through the flow until the omnibar is visible
• Switch to a different app
• Come back to the app
• Notice the app doesn’t crash
• Go to settings and change the omnibar position
• Navigate back to the browser
• Notice the app doesn’t crash

[malmstein]