Insert into database in yii2?

I am trying to insert data into MySQL table in yii2 but it gives me errors.

I have a textbox in the view. It is meant to get the data the user inputs in the textbox with name=Topic[topic]; and insert it into the database.

When I try it a different way like $topic->topic = 'good things'; it works well, but when I try to change it like $topic->topic = $topic_data['topic'];. I get this error:

Undefined index: topic`

This is the controller:

use Yii;
use common\models\LoginForm;
use common\models\Topic;
use frontend\models\ContactForm;
use frontend\models\PasswordResetRequestForm;
use frontend\models\ResetPasswordForm;
use frontend\models\SignupForm;;

use yii\base\InvalidParamException;
use yii\filters\AccessControl;
use yii\filters\VerbFilter;
use yii\web\BadRequestHttpException;
use yii\web\Controller;

    public function actionCompose()
    {
          $topic= new Topic(); 

       $topic_data = Yii::$app->request->post('Topic', []);

        $topic->creator = Yii::$app->user->identity->email;
          $topic->topic = $topic_data['topic'];

         $topic->save();
           return $this->render('compose');
    }  

And this is the view:

<?php

use yii\widgets\ListView;
use yii\data\ArrayDataProvider;
use app\models\MyProfile;
use app\models\LikeDiscussion;
use yii\widgets\ActiveForm;
use common\models\Topic;
use common\models\Comment;
use common\models\Users;
use common\models\Candidate;
use yii\widgets\Pjax;
use yii\helpers\Html;
use frontend\assets\AppAsset;

$this->title = 'My Yii Application';
?>
       <?php $form = ActiveForm::begin(); ?>

  <input type="name" class="form-control"  required="true" name="Topic[topic]" id="topic" placeholder="topic">

   <?= Html::submitButton('My Yii Application', ['class' => 'btn btn-primary', 'name' => 'login-button']) ?>   
    <?php ActiveForm::end(); ?>    

How do I fix this or is their a better way to do it?