Magento – Minicart not working in magento 2.1.2

You have keep extra slash in namespace line.

Please remove slash before class namespace in your defaultItem.php file.

Replace

<?php

namespace \XYZ\Checkout\CustomerData;

class DefaultItem extends \Magento\Checkout\CustomerData\DefaultItem
{

With

<?php

namespace XYZ\Checkout\CustomerData;

class DefaultItem extends \Magento\Checkout\CustomerData\DefaultItem
{

Somehow I found solution.

In phtml file I have added customer-data js method

require([
    'jquery',
    'Magento_Customer/js/customer-data'
    ], function($,customerData){
        $('#customerStore').change(function(){
            var store_id = $(this).val();
            var sections = ['cart'];
            $.ajax({
            url:'/customer/data/store/store_id/'+store_id,
            type:'get',
            dataType: 'html',
            beforeSend:function(){
                $('#loader').show();
            },
            success:function(response){
                customerData.invalidate(sections);
                $('#loader').hide();
                location.reload();
            }
            });
        });
    });

In my controller file Store.php I have deleted quote item from table

public function deleteQuoteItems(){
$checkoutSession = $this->getCheckoutSession();
//$quote = $this->$checkoutSession->getQuote();

 $quote_Id= $this->cart->getQuote()->getId();

//print_r($checkoutSession->getQuote()->getData());

$allItems = $checkoutSession->getQuote()->getAllVisibleItems();//returns all teh items in session
foreach ($allItems as $item) {
    $itemId = $item->getItemId();//item id of particular item
    $quoteItem=$this->getItemModel()->load($itemId);//load particular item which you want to delete by his item id
    $quoteItem->delete();
}
if(!empty($quote_Id)){
 $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
 // $quoteModel = $objectManager->create('Magento\Quote\Model\Quote');
 // $quoteModel->delete($quote_Id); 
 $resource = $objectManager->get('Magento\Framework\App\ResourceConnection');
 $connection = $resource->getConnection();
 $tableName = $resource->getTableName('quote');
    $sql = "DELETE  FROM " . $tableName." WHERE entity_id = ".$quote_Id;
    $connection->query($sql);
 }
}

Updated sections.xml. Remove vendor from action name

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Customer:etc/sections.xsd">
    <action name="module/data/store">
        <section name="cart"/>
    </action>
</config>