Magento 2 Ajax Call Error 500/503 During Checkout

I am integrating In-store pickup in Magento 2 checkout page. I have called one ajax while check page is loaded and append all stores with the In-store shipping method.

The JS file:

require([
    'jquery',
    'Magento_Checkout/js/model/shipping-rates-validator',
    'Magento_Checkout/js/model/shipping-address/form-popup-state',
    'Magento_Checkout/js/model/shipping-service',
    'Magento_Checkout/js/action/select-shipping-method',
    'Magento_Checkout/js/model/shipping-rate-registry',
    'Magento_Checkout/js/action/set-shipping-information',
    'Magento_Checkout/js/model/shipping-rate-service'
    ], function($) {
        url = $('.url').val();
        $.ajax({
            url: url+'pickupshipping/index',
        }).done(function (data) {
            $('#checkout-shipping-method-load').after('<div class="store_selectbox" style="display:none">'+data+'</div>');
        });

    $(document).on('click', '.table-checkout-shipping-method .row' ,function() {
        var sel_val = $('.all_stores').val();

        if ($('#s_method_freeshipping_freeshipping').is(':checked')) {
            instockcheck(sel_val);
            $('.store_selectbox').css('display', 'block');
        } else {
            $('.store_selectbox').css('display', 'none');
            $('#in-store-address-display').html('');
            $('#checkout-step-shipping').removeClass('sealed-content');
            $('#display-error').html('');
        }
    });

    $(document).on('click', '#shipping-method-buttons-container button', function(e){
        if ($('#s_method_freeshipping_freeshipping').is(':checked')) {
            var sel_val = $('.all_stores').val();
            var receive_later = [];
            var receive_asap = [];
            var remove_p = [];
            $('.remove_p').each(function(index, val){
                if($(this).is(':checked')) {
                    remove_p.push($(this).parent().find('.each_pro').val());
                }
            });
            $('.receive_asap').each(function(index, val){
                if($(this).is(':checked')) {
                    receive_asap.push($(this).parent().find('.each_pro').val());
                }
            });
            $('.receive_later').each(function(index, val){
                if($(this).is(':checked')) {
                    receive_later.push($(this).parent().find('.each_pro').val());
                }
            });
            $.ajax({
                data: {choose_store: sel_val, remove_p: remove_p, receive_asap: receive_asap, receive_later, receive_later},
                type: 'post',
                url: url+'pickupshipping/index/checkoutnext',
            }).done(function (data) {
            });
        }
    });

    $(document).on('change', '.all_stores', function() {
        var sel_val = $('.all_stores').val();
        instockcheck(sel_val);
    });

    function instockcheck(sel_val) {
        $.ajax({
            data: {choose_store: sel_val},
            type: 'post',
            url: url+'pickupshipping/index/checkoutinstockcheck',
        }).done(function (data) {
            if(data != 'success') {
                url1 = window.location.href+'cart';
                button_url = '<a href="'+url1+'">Goto Cart</a>';
                if ($('#s_method_freeshipping_freeshipping').is(':checked')) {
                    $('#display-error').html(data+button_url);
                    instoredisplay(sel_val);
                }

            } else {
                $('#display-error').html('');
                if ($('#s_method_freeshipping_freeshipping').is(':checked')) {
                    instoredisplay(sel_val);
                }
            }
        });
    }
    function instoredisplay(sel_val) {
        $.post(url+'pickupshipping/index/getshopdetails', {choose_store: sel_val}, function(data, textStatus, xhr) {
            $('#in-store-address-display').html(data);
        });
    }
});

This Ajax is called when choosing In-store shipping:

<?php

namespace Custom\Pickupshipping\Controller\Index;

use Magento\Framework\App\Action\Context;
//use Magento\Checkout\Model\Session as session;

class Checkoutinstockcheck extends \Magento\Framework\App\Action\Action
{
    protected $_resultPageFactory;

    public function __construct(Context $context, \Magento\Framework\View\Result\PageFactory $resultPageFactory)
    {
        $this->_resultPageFactory = $resultPageFactory;
        parent::__construct($context);
    }

    public function execute()
    {
        $product_not_instock = array();
        $shop_code = $this->getRequest()->getParam('choose_store');
        $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
        $cart = $objectManager->get('\Magento\Checkout\Model\Cart'); 
                // retrieve quote items collection
        $itemsCollection = $cart->getQuote()->getItemsCollection();
                // get array of all items what can be display directly
        $itemsVisible = $cart->getQuote()->getAllVisibleItems();
        $index = 0;
        if(!empty($itemsCollection)) {
            foreach($itemsCollection as $each_item) {
                $_product = $objectManager->create('Magento\Catalog\Model\Product')->load($each_item->getProductId());

                $inStore = $_product->getResource()->getAttribute($shop_code)->getFrontend()->getValue($_product);

                if($_product->getTypeId() == 'bundle') {
                    continue;
                }

                if(empty($inStore) || $inStore < 0) {
                    $product_not_instock[$index]['product_name'] = $_product->getName();
                    $product_not_instock[$index++]['id'] = $each_item->getProductId();
                }
            }
        }

        $msg = 'success';
        if(!empty($product_not_instock)) {
            $msg = '<div class="alert-msg-instock">Following product is not available in-store: <br/><ul>';
            foreach($product_not_instock as $each_p) {
                $action_html = '<div>
                                    <div>The quantity requested for this item exceeds the selected stores inventory.</div><br/>
                                    <div>Please select one of the following options:</div><br/>
                                    <div class="pro_container">
                                        Action Required: (Please check the box)<br/>
                                        <input type="radio" name="checkout_options_'.$each_p['id'].'" class="receive_later" value="1" />Allow an additional 0-5 days to pick up my entire order<br/>
                                        <input type="radio" name="checkout_options_'.$each_p['id'].'" class="receive_asap" value="2" />I would like to receive my items seperately as soon as the become available.<br/>
                                        <input type="radio" name="checkout_options_'.$each_p['id'].'" class="remove_p" value="3" />Remove this item from my order<br/>
                                        <input type="hidden" class="each_pro" value="'.$each_p['id'].'" />
                                    </div>
                                </div>';
                $msg .= '<li><div class="preduct_name" style="color:#000"><b>'.$each_p['product_name'].'</b></div>
                            <div>'.$action_html.'</div>
                         </li>';
            }
            $msg .= '</ul></div>';
        }


        echo $msg;
        return;
    }
}

But the Ajax call is giving 500 error.

Can anyone tell me what is the reason ?