Error while saving custom fields on place order

Hi am trying to save custom fields but am getting this error something went wrong

below is my custom fields

<div class="field sender name required">
    <label class="label" for="sender">
      <span><?= $block->escapeHtml(__('From')) ?></span>
    </label>
    <div class="control">
       <input type="text" name="sender" id="sender" value="" title="<?= $block->escapeHtmlAttr(__('From')) ?>" class="input-text" data-validate="{required:true}">
    </div>
</div>

<div class="field to required">
    <label class="label" for="to">
      <span><?= $block->escapeHtml(__('To')) ?></span>
    </label>
    <div class="control">
       <input type="text" name="to" id="to" value="" title="<?= $block->escapeHtmlAttr(__('To')) ?>" class="input-text" data-validate="{required:true}">
    </div>
</div>


<div class="field message required">
    <label class="label" for="message">
      <span><?= $block->escapeHtml(__('Your Message')) ?></span>
    </label>
    <div class="control">
       <input type="text" name="message" id="message" value="" title="<?= $block->escapeHtmlAttr(__('Your Message')) ?>" class="input-text" data-validate="{required:true}">
    </div>
</div>

My observer/event file

etc/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">

    <event name="sales_model_service_quote_submit_before">
        <observer name="mageplaza_helloworld_sales_model_service_quote_submit_before" instance="MageplazaHelloWorldObserverSalesModelServiceQuoteSubmitBeforeObserver" />
    </event>

    <event name="checkout_cart_product_add_after">
        <observer name="mageplaza_helloworld_checkout_cart_product_add_after" instance="MageplazaHelloWorldObserverCheckoutCartProductAddAfterObserver" />
    </event>
   
</config>

CheckoutCartProductAddAfterObserver.php

<?php
namespace MageplazaHelloWorldObserver;
use MagentoFrameworkEventObserver as EventObserver;
use MagentoFrameworkEventObserverInterface;
class CheckoutCartProductAddAfterObserver implements ObserverInterface
{
   
    protected $_layout;
    protected $_storeManager;
    protected $_request;
    
    public function __construct(
        MagentoStoreModelStoreManagerInterface $storeManager,
        MagentoFrameworkViewLayoutInterface $layout,
        MagentoFrameworkAppRequestInterface $request
    )
    {
        $this->_layout = $layout;
        $this->_storeManager = $storeManager;
        $this->_request = $request;
    }
 
    public function execute(EventObserver $observer)
    {
     
        $item = $observer->getQuoteItem();
        $additionalOptions = array();
        if ($additionalOption = $item->getOptionByCode('additional_options')){
            $additionalOptions = (array) unserialize($additionalOption->getValue());
        }
        $post = $this->_request->getFullActionName() == 'checkout_cart_add';
        if(is_array($post))
        {
            foreach($post as $key => $value)
            {
                if($key == '' || $value == '')
                {
                    continue;
                }
                $additionalOptions[] = [
                    'label' => $key,
                    'value' => $value
                ];
            }
        }
        if(count($additionalOptions) > 0)
        {
            $item->addOption(array(
                'code' => 'additional_options',
                'value' => serialize($additionalOptions)
            ));

             $observer->getProduct()->addCustomOption('additional_options', serialize($additionalOptions));
        }

    }
}

SalesModelServiceQuoteSubmitBeforeObserver.php

<?php
namespace MageplazaHelloWorldObserver;
use MagentoFrameworkEventObserver as EventObserver;
use MagentoFrameworkEventObserverInterface;
class SalesModelServiceQuoteSubmitBeforeObserver implements ObserverInterface
{
    private $quoteItems = [];
    private $quote = null;
    private $order = null;
   
    public function execute(EventObserver $observer)
    {
        $this->quote = $observer->getQuote();
        $this->order = $observer->getOrder();
        foreach($this->order->getItems() as $orderItem)
        {
            if(!$orderItem->getParentItemId() && $orderItem->getProductType() == MagentoCatalogModelProductType::TYPE_SIMPLE)
            {
                if($quoteItem = $this->getQuoteItemById($orderItem->getQuoteItemId())){
                    if ($additionalOptionsQuote = $quoteItem->getOptionByCode('additional_options'))
                    {
                       
                        if($additionalOptionsOrder = $orderItem->getProductOptionByCode('additional_options'))
                        {
                            $additionalOptions = array_merge($additionalOptionsQuote, $additionalOptionsOrder);
                        }
                        else
                        {
                            $additionalOptions = $additionalOptionsQuote;
                        }
                        if(count($additionalOptions) > 0)
                        {
                            $options = $orderItem->getProductOptions();
                            $options['additional_options'] = unserialize($additionalOptions->getValue());
                            $orderItem->setProductOptions($options);
                        }
                    }
                }
            }
        }
    }
    private function getQuoteItemById($id)
    {
        if(empty($this->quoteItems))
        {
          
            foreach($this->quote->getItems() as $item)
            {
                
                if(!$item->getParentItemId() && $item->getProductType() == MagentoCatalogModelProductType::TYPE_SIMPLE)
                {
                    $this->quoteItems[$item->getId()] = $item;
                }
            }
        }
        if(array_key_exists($id, $this->quoteItems))
        {
            return $this->quoteItems[$id];
        }
        return null;
    }
}

Any solution for this .. thanks

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