Yii::$app->session->setFlash() is not working - yii2

Question

Following code in SiteController works fine with echo, but not with yii-method setflash(). Maybe, I have to reconfigure my config-file(main-local.php)?
Any other ideas how to keep setflash() doing its job?
public function actionScript() { //A new method, programmed by Thomas Kipp
$model = new myScriptForm();
$fileName = 'file';
$uploadPath = Yii::getAlias('#uploading');
if (isset($_FILES[$fileName])) {
$file = \yii\web\UploadedFile::getInstanceByName($fileName);
if ($file->saveAs($uploadPath . '/' . $file->name)) {
echo"<script>alert('Hallo');</script>";
//echo \yii\helpers\Json::encode($file);
}
}
if ($model->load(Yii::$app->request->post()) && $model->validate()) {
$model->fileImage = UploadedFile::getInstance($model, 'fileImage');
$model->avatar = UploadedFile::getInstances($model, 'avatar');
if ($model->fileImage) {
Yii::$app->session->setFlash('success', 'File has been uploaded');
echo "<font size='4'><br><br><br><center>File <font color='red'> "
. "$model->fileImage<font color='black'> successfully uploaded."
. "<br>It's available in folder 'uploadedfiles' </font></font color></center>";
$model->fileImage->saveAs(Yii::getAlias('#uploadedfilesdir/' . $model->fileImage->baseName . '.' . $model->fileImage->extension));
} else {
Yii::$app->session->setFlash('error', 'There has nothing to be uploaded');
echo"<font size='4'><br><br><br><center>No Upload-file selected.<br>"
. "Nothing moved into folder 'uploadedfiles'</font></center>";
}
if ($model->avatar) {
$counter = 0;
foreach ($model->avatar as $avatar) {
Yii::$app->session->setFlash('success', 'Avatar has been uploaded');
echo "<font size='4'><br><center>File <font color='red'> "
. "$avatar<font color='black'> successfully uploaded."
. "<br>It's available in folder 'uploadedfiles' </font></font color></center>";
$avatar->saveAs(Yii::getAlias('#uploadedfilesdir/' . $avatar->baseName . $counter . '.' . $avatar->extension));
$counter++;
}
} else {
Yii::$app->session->setFlash('error', 'There has nothing to be uploaded');
echo"<font size='4'><br><center>No Upload-file selected.<br>"
. "Nothing moved into folder 'uploadedfiles' </font></center>";
}
return $this->render('myScript', ['model' => $model]);
} else {
return $this->render('myScript_Formular', ['model' => $model]);
}
}

Answer 1

once you have assign the value with eg:
Yii::$app->session->setFlash('error', 'There has nothing to be uploaded');
be sure that in your view eg: myScript.php
you properly echo the value setted
<?= Yii::$app->session->getFlash('error'); ?>