Yii2 relations NULL Foreign key How to do CREATE/UPDATE correctly?

Question

Yii

People, such trouble:
There is a groups table
... ... And there is a files table ...
'file_id' => $this->integer(),

'id' => $this->primaryKey()->notNull()->unique(),
'filename' => $this->string(),

...

$this->addForeignKey(
            'file-id',
            'groups',
            'file_id',
            'files',
            'id',
            'CASCADE'
        );

Gii generated:

class Groups extends \yii\db\ActiveRecord
{
...
[['file_id'], 'default', 'value' => null],
[['file_id'], 'integer'],
[['file_id'], 'exist', 'skipOnError' => true, 'targetClass' => Files::className(), 'targetAttribute' => ['file_id' => 'id']],
...
public function getFile()
{
    return $this->hasOne(Files::className(), ['id' => 'file_id']);
}
...
class Files extends \yii\db\ActiveRecord
{
...
public function getGroups()
{
    return $this->hasMany(Groups::className(), ['file_id' => 'id']);
}
...
public function actionMyaction($id)
{
...
$model = $this->findModel($id);  
...
return $this->render('Myaction', [
    'model' => $model,
]);

Now, in the Myaction form, if there is a non-NULL file_id value in the Groups table and a corresponding value in the Files id table, I can get the filename value:

<?= $form->field($model->file, 'filename')->textInput(); ?>

The trouble starts if there is no corresponding value in the Files table
Call to a member function isAttributeRequired() on null

I assume that there should be New Files() somewhere, but where?
Please Help!