"#windowIcon was sent to nil" while updating theme on image startup

[hernanmd]

Bug description When a new image is opened, the first

To Reproduce

1. Have a startup.st in General Preferences Folder with contents similar to https://github.com/hernanmd/pharo-startup-settings/blob/main/startup.st
2. Open the image
3. See the Debugger error

Screenshots

Version information:

• OS: macOS
• Pharo Version: Pharo 13.0.0 Build information: Pharo-13.0.0+SNAPSHOT.build.301.sha.4bac191937fcf397a09f8a4ed2cebc4d5ae72dfe (64 Bit)

Additional context The line causing the problem is

		...
				PolymorphSystemSettings uiThemeClass: PharoLightTheme.
                ...