Why does PhpSpreadsheet fail to identify my Xlsx file in public://? - drupal-8

Question

I'm working on a Drupal module that imports a Xlsx file and inserts the rows to database. Everything works fine, except PhpSpreadsheet throws the error "PHP message: Uncaught PHP Exception InvalidArgumentException: "File "" does not exist." at File.php line 137" when my uploaded file is in public. This error doesn't happen when the file is in base_path(). Line 137 throws an exception saying it's not a file, but \dpm(is_file($uri)) returns TRUE. Why is this happening? Below is my code:
public function submitForm(array &$form, FormStateInterface $form_state) {
$connection = \Drupal::database();
$fid = $form_state->getValue("xlsx_upload");
$input = File::load(reset($fid));
$input->setPermanent();
$uri = \Drupal::service('file_system')->realpath($input->getFileUri());
//$uri = '/home/joe/file.xslx'; <--- this works perfectly.
$reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReaderForFile($uri);
$spreadsheet = $reader->load($filename);
$dataArray = $spreadsheet->getActiveSheet()->toArray();
$connection->truncate("mitacan")->execute();
$connection->query("SET NAMES utf8");
foreach ($dataArray as $dataItem) {
$connection->insert('mitacan')
->fields([
'stokkodu' => $dataItem[0],
'logo' => $dataItem[1],
'resim1' => $dataItem[2],
'grupkodu' => $dataItem[3],
'grupadi' => $dataItem[4],
'grupindex' => $dataItem[5],
'paketadedi' => $dataItem[6],
])
->execute();
}
drupal_set_message('Excel verisi veritabanına eklendi.');
}

Answer 1

$spreadsheet = $reader->load($filename);
$filename is undefined. Use $uri instead.