The method findById() does not exist on hiqdev\billing\hiapi\customer\CustomerRepository. Since you implemented __call, consider adding a @method annotation.
(
Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation
Accessing username on the interface yii\web\IdentityInterface suggest that you code against a concrete implementation. How about adding an instanceof check?