Magento 2 declaring modifier PHP Fatal Error: null given in $modifiers

Question

I get this php error:

PHP Fatal error: Uncaught TypeError: Argument 2 passed to Magento\Ui\DataProvider\Modifier\Pool::__construct() must be of the type array, null given, called in /vendor/magento/framework/ObjectManager/Factory/AbstractFactory.php on line 111 and defined in /vendor/magento/module-ui/DataProvider/Modifier/Pool.php:34

Vendor/Module/etc/adminhtml/di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
        <virtualType name="Vendor\Module\Ui\DataProvider\Item\Form\Modifier\Pool" type="Magento\Ui\DataProvider\Modifier\Pool">
            <arguments>
                <argument name="modifiers" xsi:type="array">
                    <item name="test-modifier" xsi:type="array">
                        <item name="class" xsi:type="string">Vendor\Module\Ui\DataProvider\Item\Form\Modifier\TestModifier</item>
                        <item name="sortOrder" xsi:type="number">20</item>
                    </item>
                </argument>
            </arguments>
        </virtualType>
    </config>

Vendor/Module/Ui/DataProvider/Item/Form/DataProvider.php

    <?php
namespace Vendor\Module\Ui\DataProvider\Item\Form;

    use Vendor\Module\Model\ResourceModel\Item\CollectionFactory;
    use Magento\Ui\DataProvider\AbstractDataProvider;
    use Magento\Ui\DataProvider\Modifier\ModifierInterface;
    use Magento\Ui\DataProvider\Modifier\PoolInterface;

    class DataProvider extends AbstractDataProvider
    { 
        /**
         * @var PoolInterface
         */
        private $pool;

        /**
         * Construct
         *
         * @param string $name
         * @param string $primaryFieldName
         * @param string $requestFieldName
         * @param CollectionFactory $collection
         * @param PoolInterface poolInterface
         * @param array $meta
         * @param array $data
         */
        public function __construct(
            $name,
            $primaryFieldName,
            $requestFieldName,
            CollectionFactory $collection,
            PoolInterface $poolInterface,
            array $meta = [],
            array $data = []
        ) {
            parent::__construct($name, $primaryFieldName, $requestFieldName, $meta, $data);
            $this->collection = $collectionFactory->create();
            $this->pool = $poolInterface;
        }

        public function getData()
        {
            /** @var ModifierInterface $modifier */
            foreach ($this->pool->getModifiersInstances() as $modifier) {
                $this->data = $modifier->modifyData($this->data);
            }

            return $this->data;
        }

        public function getMeta()
        {
            $meta = parent::getMeta();

            /** @var ModifierInterface $modifier */
            foreach ($this->pool->getModifiersInstances() as $modifier) {
                $meta = $modifier->modifyMeta($meta);
            }

            return $meta;
        }
    }

Vendor/Module/Ui/DataProvider/Item/Form/Modifier/TestModifier.php

    <?php
namespace Vendor\Module\Ui\DataProvider\Item\Form\Modifier;

    use Magento\Ui\DataProvider\Modifier\ModifierInterface;

    class TestModifier implements ModifierInterface
    {

        public function modifyMeta(array $meta)
        {
            $meta['test_fieldset_name'] = [
                'arguments' => [
                    'data' => [
                        'config' => [
                            'label' => __('Label For Fieldset'),
                            'sortOrder' => 50,
                            'collapsible' => true
                        ]
                    ]
                ],
                'children' => [
                    'test_field_name' => [
                        'arguments' => [
                            'data' => [
                                'config' => [
                                    'formElement' => 'select',
                                    'componentType' => 'field',
                                    'options' => [
                                        ['value' => 'test_value_1', 'label' => 'Test Value 1'],
                                        ['value' => 'test_value_2', 'label' => 'Test Value 2'],
                                        ['value' => 'test_value_3', 'label' => 'Test Value 3'],
                                    ],
                                    'visible' => 1,
                                    'required' => 1,
                                    'label' => __('Label For Element')
                                ]
                            ]
                        ]
                    ]
                ]
            ];
            return $this->meta;
        }

        /**
         * {@inheritdoc}
         */
        public function modifyData(array $data)
        {
            return $data;
        }
    }

Vendor/Module/view/adminhtml/ui_component/item_form.xml

<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
    <argument name="data" xsi:type="array">
        <item name="js_config" xsi:type="array">
            <item name="provider" xsi:type="string">item_form.item_form_data_source</item>
            <item name="deps" xsi:type="string">item_form.item_form_data_source</item>
        </item>
        <item name="config" xsi:type="array">
            <item name="namespace" xsi:type="string">item_form</item>
            <item name="dataScope" xsi:type="string">data</item>
            <item name="template" xsi:type="string">templates/form/collapsible</item>
            <item name="childrenFromMeta" xsi:type="boolean">true</item>
        </item>
        <item name="label" xsi:type="string" translate="true">Item Form</item>
        <item name="buttons" xsi:type="array">
            <item name="back" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Edit\BackButton</item>
            <item name="delete" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Edit\DeleteButton</item>
            <item name="save" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Edit\SaveButton</item>
            <item name="save_and_continue" xsi:type="string">Vendor\Module\Block\Adminhtml\Item\Edit\SaveAndContinueButton</item>
        </item>
    </argument>
    <dataSource name="item_form_data_source">
        <argument name="dataProvider" xsi:type="configurableObject">
            <argument name="class" xsi:type="string">Vendor\Module\Ui\DataProvider\Item\Form\DataProvider</argument>
            <argument name="name" xsi:type="string">item_form_data_source</argument>
            <argument name="primaryFieldName" xsi:type="string">itemID</argument>
            <argument name="requestFieldName" xsi:type="string">itemID</argument>
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="submit_url" xsi:type="url" path="*/*/save"/>
                    <item name="validate_url" xsi:type="url" path="*/*/validate"/>
                </item>
            </argument>
        </argument>
        <argument name="data" xsi:type="array">
            <item name="js_config" xsi:type="array">
                <item name="component" xsi:type="string">Magento_Ui/js/form/provider</item>
            </item>
        </argument>
    </dataSource>
</form>

Is the code above correct? Has anyone an idea why I get the PHP Fatal Error "null" given on $modifiers?

EDIT: I've removed the generated/code folder. The result is the following exception: Missing required argument $modifiers of Magento\Ui\DataProvider\Modifier\Pool.

And when I run php bin/magento setup:di:compile I get the PHP Fatal Error: null given in $modifiers as mentioned above.

EDIT 2: It doesn't matter if the argument within di.xml is called modifiers or anything else, behavior stays the same as described above.

Answer 1

Finally, this worked for me:

changing Vendor/Module/etc/adminhtml/di.xml to

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
        <virtualType name="Vendor\Module\Ui\DataProvider\Item\Form\Modifier\Pool" type="Magento\Ui\DataProvider\Modifier\Pool">
            <arguments>
                <argument name="modifiers" xsi:type="array">
                    <item name="test-modifier" xsi:type="array">
                        <item name="class" xsi:type="string">Vendor\Module\Ui\DataProvider\Item\Form\Modifier\TestModifier</item>
                        <item name="sortOrder" xsi:type="number">20</item>
                    </item>
                </argument>
            </arguments>
        </virtualType>
        <type name="Vendor\Module\Ui\DataProvider\Item\Form\DataProvider">
        <arguments>
            <argument name="pool" xsi:type="object">Vendor\Module\Ui\DataProvider\Item\Form\Modifier\Pool</argument>
        </arguments>
    </type>
    </config>

and changing Vendor/Module/Ui/DataProvider/Item/Form/DataProvider.php to

<?php
namespace Vendor\Module\Ui\DataProvider\Item\Form;

    use Vendor\Module\Model\ResourceModel\Item\CollectionFactory;
    use Magento\Ui\DataProvider\AbstractDataProvider;
    use Magento\Ui\DataProvider\Modifier\ModifierInterface;
    use Magento\Ui\DataProvider\Modifier\PoolInterface;

    class DataProvider extends AbstractDataProvider
    { 
        /**
         * @var PoolInterface
         */
        private $pool;

        /**
         * Construct
         *
         * @param string $name
         * @param string $primaryFieldName
         * @param string $requestFieldName
         * @param CollectionFactory $collection
         * @param PoolInterface $pool
         * @param array $meta
         * @param array $data
         */
        public function __construct(
            $name,
            $primaryFieldName,
            $requestFieldName,
            CollectionFactory $collection,
            PoolInterface $pool,
            array $meta = [],
            array $data = []
        ) {
            parent::__construct($name, $primaryFieldName, $requestFieldName, $meta, $data);
            $this->collection = $collection->create();
            $this->pool = $pool;
        }

        public function getData()
        {
            /** @var ModifierInterface $modifier */
            foreach ($this->pool->getModifiersInstances() as $modifier) {
                $this->data = $modifier->modifyData($this->data);
            }

            return $this->data;
        }

        public function getMeta()
        {
            $meta = parent::getMeta();

            /** @var ModifierInterface $modifier */
            foreach ($this->pool->getModifiersInstances() as $modifier) {
                $meta = $modifier->modifyMeta($meta);
            }

            return $meta;
        }
    }

Explanation: The pool now can be injected correctly to the DataProvider because of the correct naming of the arguments.

Answer 2

As per my knowledge and past experience with this error (PHP Fatal error: Uncaught TypeError: Argument x passed to ...).

Whenever we add dependency injection class to constructor for respective module class file, if we get above mention or similar error,

Then need to do

For Magento 2.0.x and 2.1.x

You need to remove var/generation folder content

For Magento 2.2.x ( generation folder is moved out from var folder and renamed as generated in Magento 2.2.x )

You need to remove generated/code folder content.

Then refresh the page, It will solve the above concern.