Form field name not rendered after submit the form

Question

I created a form and designed it by referred this link. It worked fine. I used the following code to display a textbox in my form

public function buildForm(array $form, FormStateInterface $form_state) {    
    $form['searchtext'] = array(
          '#type' => 'textfield',
          '#title' => t('Keyword Search:'),
          //'#default_value' => (isset($record['searchtext'])) ? $record['searchtext']:'',
          '#attributes' => array('placeholder' => t('Enter keyword to assist locating notices relevant to your vehicle')),
        );
    $form['#theme'] = ['notices_search_form'];
    //echo "before submit: <pre>";print_r($form);echo "</pre>";
    return $form;
}

and it is rendered correctly as i expected as below (it has name, id and everything)

<input placeholder="Enter keyword to assist locating notices relevant to your vehicle" data-drupal-selector="edit-searchtext" type="text" id="edit-searchtext" name="searchtext" value="" size="60" maxlength="128" class="form-text">

But after submitting the form, I stop the redirection and load the form with the value entered while submitting form using following code

public function submitForm(array &$form, FormStateInterface $form_state) {
    $fieldvalues=$form_state->getValues();
    $form['searchtext']['#value'] = (isset($fieldvalues['searchtext'])) ? $fieldvalues['searchtext']:'';
    $form_state->disableRedirect();
    return $form;
  }

But this time form loads without name or id as below

<input placeholder="Enter keyword to assist locating notices relevant to your vehicle" type="text" value="sdfdf" size="60" maxlength="128" class="form-text">

I am new to drupal. Someone suggest me what am i missing?

Edit 1

I tried to place the name in the array like below and it is worked fine.

$form['searchtext'] = array(
      '#name' => 'searchtext',
      '#type' => 'textfield',
      '#title' => t('Keyword Search:'),
      //'#default_value' => (isset($record['searchtext'])) ? $record['searchtext']:'',
      '#attributes' => array('placeholder' => t('Enter keyword to assist locating notices relevant to your vehicle')),
    );

But setting 'true' to "#default_value" for checkbox field is not checking while rendering it.