YII2 Call to a member function isAttributeRequired() on a non-object - yii2

Question

I am new with Yii2, im trying to make sales transaction, but i have problem for dropdownlist in detail section.
for this module, i have 3 tables :
1. for header, table salesheader
2. for detail, table salesdetail
3. for item, table item
this is my code :
on controller :
public function actionUpdate($id)
{
$model = $this->findModel($id); //find appropriate record in salesheader
//$modeldetail = salesdetail::findModel($id);
$modeldetail = new salesdetail();
if ($model->load(Yii::$app->request->post()) && $model->save()) {
return $this->redirect(['index']);
} else {
$item = item::find()->select(["concat('[',code,'] - ', name) as item", "id"])->indexBy('id')->column();
$detail = salesdetail::find()->select(["*"])->all(); //find appropriate record in salesdetail
return $this->render('update', [
'model' => $model,
'modeldetail' => $modeldetail,
'item' => $item,
'detail' => $detail
]);
}
}
on salesheader model
namespace app\models;
use Yii;
class salesheader extends \yii\db\ActiveRecord
{
public static function tableName()
{
return 'salesheader';
}
public function rules()
{
return [
[['partnerId', 'date', 'term', 'name'], 'required'],
[['name', 'invNumber'], 'string'],
[['partnerId', 'term'], 'integer']
];
}
public function attributeLabels()
{
return [
'partnerId' => 'Customer',
'date' => 'Date',
'invNumber' => 'Inv. Number',
'term' => 'Term',
'name' => 'Name'
];
}
on salesdetail model
namespace app\models;
use Yii;
class salesdetail extends \yii\db\ActiveRecord
{
public static function tableName()
{
return 'salesdetail';
}
public function rules()
{
return [
[['itemId', 'qty', 'price'], 'required'],
[['unit'], 'string']
];
}
/**
* #inheritdoc
*/
public function attributeLabels()
{
return [
'itemId' => 'Item',
];
}
}
on my view
<?= $form->field($detail, 'itemId')->dropDownList($item)->label(false) ?>
and when open the page in browser, i got this error "Call to a member function isAttributeRequired() on a non-object"
and if i change this line on controller
//$modeldetail = salesdetail::findModel($id);
$modeldetail = new salesdetail();
to this one
$modeldetail = salesdetail::findModel($id);
//$modeldetail = new salesdetail();
then the error would be "Call to undefined method app\models\salesdetail::findModel()"
can somebody tell me what am i doing wrong ?
and how to get appropriate data from salesdetail based on salesheader and put it on dropdownlist ?
Thank you for ur help