Tested with:
1. Sencha Architect 4.2.7 and 4.2.8
2. Sencha Cmd from 7.0 to 7.2
3. ExtJS 7.0 to 7.2 both Modern/Classic

Steps to reproduce:
1. Create blank project
2. Add button and save project
3. Add new UI to button and look to Cmd Output:

[INF] -----------------------
[LOG] Fashion build starting for /Users/user/extjs/test_button/build/temp/development/MyApp/sass/MyApp-all.scss
[INF] Waiting for changes...
[ERR] unknown definition for mixin named undefined : /Users/user/extjs/test_button/packages/local/theme-triton-1f740189-602e-4693-85f3-818ce21e564c/sass/src/_architect_src.scss:2
[LOG] Build error for /Users/user/extjs/test_button/build/temp/development/MyApp/sass/MyApp-all.scss
From this point it's impossible to recover your project unless you manually remove broken scss files, that makes UI functionality absolutely broken and useless