Magento – Magento2: Custom source model – Class does not exist

I have created a multi-select EAV Attribute for my customers with a custom source model. However, whenever I browse to the customers management page or call getSource on the attribute, I get a class does not exist error for the source model. Am I missing something?

Sample exception:

Exception #0 (ReflectionException): Class Wizbusiness/CustomerSubscriptions/Model/Config/Source/ProductTags does not exist

Exception #0 (ReflectionException): Class Wizbusiness/CustomerSubscriptions/Model/Config/Source/ProductTags does not exist
#0 <path>/vendor/magento/framework/Code/Reader/ClassReader.php(19): ReflectionClass->__construct('Wizbusiness/Cus...')
#1 <path>/vendor/magento/framework/ObjectManager/Definition/Runtime.php(44): Magento\Framework\Code\Reader\ClassReader->getConstructor('Wizbusiness/Cus...')
#2 <path>/vendor/magento/framework/ObjectManager/Factory/Dynamic/Developer.php(71): Magento\Framework\ObjectManager\Definition\Runtime->getParameters('Wizbusiness/Cus...')
#3 <path>/vendor/magento/framework/ObjectManager/ObjectManager.php(57): Magento\Framework\ObjectManager\Factory\Dynamic\Developer->create('Wizbusiness/Cus...', Array)
#4 <path>/vendor/magento/framework/Validator/UniversalFactory.php(36): Magento\Framework\ObjectManager\ObjectManager->create('Wizbusiness/Cus...', Array)
#5 <path>/vendor/magento/module-eav/Model/Entity/Attribute/AbstractAttribute.php(538): Magento\Framework\Validator\UniversalFactory->create('Wizbusiness/Cus...')
#6 <path>/vendor/magento/module-customer/Model/AttributeMetadataConverter.php(66): Magento\Eav\Model\Entity\Attribute\AbstractAttribute->getSource()
...

For your reference module files:
app/code/Wizbusiness/CustomerSubscriptions/Model/Config/Source/ProductTags.php:

<?php
namespace Wizbusiness\CustomerSubscriptions\Model;

/**
 * Tags copied from products
 *
 * @author Wizbusiness
 */
class ProductTags extends \Magento\Eav\Model\Entity\Attribute\Source\AbstractSource
{


    public function toArray()
    {
        return [];
    }

    /**
     * Options getter
     * @return array
     */
    final public function toOptionArray()
    {
         $objectManager = \Magento\Framework\App\ObjectManager::getInstance();
         $model = $objectManager->get('Magento\Catalog\Model\Product');
         $attribute = $model->getAttribute('tags');
         return $attribute->getAllOptions();
    }

    /**
     * @return array
     */
    public function getAllOptions()
    {
        return $this->toOptionArray();
    }

     /**
     * Get a text for option value
     *
     * @param string|integer $value
     * @return string|bool
     */
    public function getOptionText($value)
    {
        foreach ($this->getAllOptions() as $option) {
            if ($option['value'] == $value) {
                return $option['label'];
            }
        }
        return false;
    }

    /**
     * Retrieve flat column definition
     *
     * @return array
     */
    public function getFlatColumns()
    {
        $attributeCode = $this->getAttribute()->getAttributeCode();
        return [
            $attributeCode => [
                'unsigned' => false,
                'default' => null,
                'extra' => null,
                'type' => Table::TYPE_INTEGER,
                'nullable' => true,
                'comment' => 'Custom Attribute Options  ' . $attributeCode . ' column',
            ],
        ];
    }
}

app/code/Wizbusiness/CustomerSubscriptions/Setup/InstallData.php:

<?php
namespace Wizbusiness\CustomerSubscriptions\Setup;

use Magento\Customer\Model\Customer;
use Magento\Customer\Setup\CustomerSetup;
use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Eav\Model\Entity\Attribute\Backend\ArrayBackend;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Catalog\Model\Resource\Eav\Attribute;

class InstallData implements InstallDataInterface
{

    /**
     * @var CustomerSetupFactory
     */
    private $customerSetupFactory;

    public function __construct(CustomerSetupFactory $customerSetupFactory)
    {
        $this->customerSetupFactory = $customerSetupFactory;
    }

    /**
     * Upgrades data for a module
     *
     * @param ModuleDataSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     */

    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $setup->startSetup();

        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);

        /** @var CustomerSetup $customerSetup */
        $customerSetup->addAttribute(
            Customer::ENTITY,
            'subscribedtags',
            [
                'type' => 'text',
                'input' => 'multiselect',
                'label' => 'Subscribed Tags',
                'required' => 0,
                'system' => 0, // <-- important, otherwise values aren't saved.
                               // @see Magento\Customer\Model\Metadata\CustomerMetadata::getCustomAttributesMetadata()
                'position' => 100,
                'filterable' => true,
                'source' => 'Wizbusiness/CustomerSubscriptions/Model/Config/Source/ProductTags',
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
            ]
        );

        $used_in_forms=array();
        $used_in_forms[]="adminhtml_customer";
        $used_in_forms[]="checkout_register";
        $used_in_forms[]="customer_account_create";
        $used_in_forms[]="customer_account_edit";
        $used_in_forms[]="adminhtml_checkout";
        $customerSetup->getEavConfig()->getAttribute('customer', 'subscribedtags')
            ->setData('used_in_forms', $used_in_forms)
            ->setData("is_used_for_customer_segment", true)
            ->setData("is_user_defined", 1)
            ->save();
        $setup->endSetup();
    }
}

app/code/Wizbusiness/CustomerSubscriptions/etc/module.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
                <module name="Wizbusiness_CustomerSubscriptions" setup_version="0.0.11" />
</config>

app/code/Wizbusiness/CustomerSubscriptions/registration.php:

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Wizbusiness_CustomerSubscriptions',
    __DIR__
);