The type SilverStripe\UserForms\M...ableFormField\FieldList was not found. Maybe you did not declare it correctly or list all dependencies?
The issue could also be caused by a filter entry in the build configuration.
If the path has been excluded in your configuration, e.g. excluded_paths:["lib/*"],
you can move it to the dependency path list as follows:
The expression return $fields returns the type SilverStripe\Forms\FieldList which is incompatible with the documented return type SilverStripe\UserForms\M...ableFormField\FieldList.
The property CheckedDefault does not exist on SilverStripe\UserForms\M...mField\EditableCheckbox. Since you implemented __get, consider adding a @property annotation.