Yii2-Unable to send image name via client interface - yii2

Question

I am working on Yii2. I have developed an API which will save the incoming records from the client. A client can send an image so I am saving its name and then uploading in the folder. Along with the name of the image I am also sending data with it. All the data except the image name are saved. Below is my API call.
public function actionAddnew()
{
$fp = fopen('preinstall.txt', 'w+');
fwrite($fp, file_get_contents('php://input'));
fclose($fp);
$inputs = json_decode(file_get_contents('php://input'));
return PreInstallations::saveAll($inputs);
}
The function saveAll is
public static function saveAll($inputs)
{
$coutner = 0;
$arr_status = [];
foreach ($inputs as $input) {
$s = new PreInstallations;
foreach ((array)$input as $key => $value) {
if ($key != 'image_names') {
if ($s->hasAttribute($key)) {
$s->$key = $value;
}
}
}
$user = Yii::$app->user;
if (isset($input->auth_key) && Users::find()->where(['auth_key' => $input->auth_key])->exists()) {
$user = Users::find()->where(['auth_key' => $input->auth_key])->one();
}
$s->created_by = $user->id;
if (PreInstallations::find()->where(['ref_no' => $input->ref_no])->exists()) {
$arr_status[] = ['install_id' => $input->install_id, 'status' => 2, 'messages' => "Ref # Already exists"];
continue;
}
$s->sync_date = date('Y-m-d H:i:s  ');
if ($s->save()) {
if ($s->pre_install_status == 'Pre Installed') {
Meters::change_status_byinstall($s->meter_msn, Meters::$status_titles[8]);
}
$arr_status[] = ['install_id' => $input->install_id, 'status' => 1];
$coutner++;
if (isset($input->site_images_name)) {
foreach ($input->site_images_name as $img) {
$image2 = new PreInstallationImagesSite;
$image2->image_name = $img->image_name;
$image2->pre_install_id = $s->id;
$image2->save();
}
}
}
else {
$arr_status[] = ['install_id' => $input->install_id, 'status' => 0, 'messages' => $s->errors];
}
}
return ['status' => 'OK', 'details' => $arr_status, 'records_saved' => $coutner];
}
Below part of above code saves the images name
if (isset($input->site_images_name)) {
foreach ($input->site_images_name as $img) {
$image2 = new PreInstallationImagesSite;
$image2->image_name = $img->image_name;
$image2->pre_install_id = $s->id;
$image2->save();
}
}
For testing purpose API call is made from POSTMAN with below details
[{
"ref_no": "20371521444700U",
"meter_msn" :"002999001189",
"billing_msn" : "74516" ,
"latitude": "36.2703169",
"longitude": "78.178266",
"tarrif": "07",
"s_load": "07",
"customer_id" :"37010673625",
"pre_install_status": "Pre Installed",
"site_issues" : "No issue",
"install_id" : "20371521444700U_1521263491",
"created_date": "2018-03-17 10:12:12",
"consumer_name": "HDA PUMPING STATION SEVERAGE UNO.8 LATIFABAD HYD",
"consumer_address" :"HDA PUMPING STATION SEVERAGE UNO.8 LATIFABAD HYD",
"auth_key": "key",// replaced the original auth key with key
"ct_ratio": "200/5",
"ct_ratio_quantity":"4",
"cable_type":"37/83",
"cable_length":"11",
"atb_installed": "Yes",
"meter_type":"L.T.TOU",
"site_images_name":["20371521444700U_1522299780_site_1.jpg"]
}]
When I Send the request I am getting an error
{
"name": "PHP Notice",
"message": "Trying to get property of non-object",
"code": 8,
"type": "yii\\base\\ErrorException",
"file": "E:\\xampp\\htdocs\\inventory-web\\common\\models\\PreInstallations.php",
"line": 137,
"stack-trace": [
"#0 E:\\xampp\\htdocs\\inventory-web\\common\\models\\PreInstallations.php(137): yii\\base\\ErrorHandler->handleError(8, 'Trying to get p...', 'E:\\\\xampp\\\\htdocs...', 137, Array)",
"#1 E:\\xampp\\htdocs\\inventory-web\\api\\modules\\v1\\controllers\\PreinstallationController.php(36): common\\models\\PreInstallations::saveAll(Array)",
"#2 [internal function]: api\\modules\\v1\\controllers\\PreinstallationController->actionAddnew()",
"#3 E:\\xampp\\htdocs\\inventory-web\\vendor\\yiisoft\\yii2\\base\\InlineAction.php(57): call_user_func_array(Array, Array)",
"#4 E:\\xampp\\htdocs\\inventory-web\\vendor\\yiisoft\\yii2\\base\\Controller.php(156): yii\\base\\InlineAction->runWithParams(Array)",
"#5 E:\\xampp\\htdocs\\inventory-web\\vendor\\yiisoft\\yii2\\base\\Module.php(523): yii\\base\\Controller->runAction('addnew', Array)",
"#6 E:\\xampp\\htdocs\\inventory-web\\vendor\\yiisoft\\yii2\\web\\Application.php(102): yii\\base\\Module->runAction('v1/preinstallat...', Array)",
"#7 E:\\xampp\\htdocs\\inventory-web\\vendor\\yiisoft\\yii2\\base\\Application.php(380): yii\\web\\Application->handleRequest(Object(yii\\web\\Request))",
"#8 E:\\xampp\\htdocs\\inventory-web\\api\\web\\index.php(35): yii\\base\\Application->run()",
"#9 {main}"
]
}
The line 137 is $image2->image_name = $img->image_name;.
The Image model is
public function rules()
{
return [
[['pre_install_id'], 'required'],
[['pre_install_id', 'image_upload_flag'], 'integer'],
[['image_name'], 'string', 'max' => 255],
[['pre_install_id'], 'exist', 'skipOnError' => true, 'targetClass' => PreInstallations::className(), 'targetAttribute' => ['pre_install_id' => 'id']],
];
}
/**
* #inheritdoc
*/
public function attributeLabels()
{
return [
'id' => 'ID',
'pre_install_id' => 'Pre Install ID',
'image_name' => 'Image Name',
'image_upload_flag' => 'Image Upload Flag',
];
}
I must be missing something that I don't know. Any help would be highly appreciated.