Steps to reproduce:

1. You have a model and addExpression which by nature is non-editable.
2. You for example relate to that model with a hasOne relation, and import that expression as a field

Error TypeError: Atk4\Data\Field::isEditable(): Return value must be of type bool, array returned
in Model:730

The issue is in line 734, as isEditable is not existing as a property for SqlExpressionField.