Magento 2 : Product grid serializer issue

you have to add this getProductsPosition method in your model .i had same issue then i customize below code

 public function getProductsPosition()
{
    if (!$this->getId()) {
        return [];
    }
    $array = $this->getData('products_position');
    if ($array === null) {
        $temp = $this->getData('product_id');

        for ($i = 0; $i < sizeof($this->getData('product_id')); $i++) {
            $array[$temp[$i]] = 0;
        }
        $this->setData('products_position', $array);
    }
    return $array;
}

Update your assign-products.js

define([
    'mage/adminhtml/grid'
], function () {
    'use strict';

    return function (config) {
        var selectedProducts = config.selectedProducts,
            categoryProducts = $H(selectedProducts),
            gridJsObject = window[config.gridJsObjectName],
            tabIndex = 1000;

        $('in_category_products').value = Object.toJSON(categoryProducts);

        /**
         * Register Category Product
         *
         * @param {Object} grid
         * @param {Object} element
         * @param {Boolean} checked
         */
        function registerCategoryProduct(grid, element, checked) {
            if (checked) {

                 categoryProducts.set(element.value, '');
            } else {                
                categoryProducts.unset(element.value);
            }
            $('in_category_products').value = Object.toJSON(categoryProducts);
            grid.reloadParams = {
                'selected_products[]': categoryProducts.keys()
            };
        }

        /**
         * Click on product row
         *
         * @param {Object} grid
         * @param {String} event
         */
        function categoryProductRowClick(grid, event) {
            var trElement = Event.findElement(event, 'tr'),
                isInput = Event.element(event).tagName === 'INPUT',
                checked = false,
                checkbox = null;

            if (trElement) {
                checkbox = Element.getElementsBySelector(trElement, 'input');

                if (checkbox[0]) {
                    checked = isInput ? checkbox[0].checked : !checkbox[0].checked;
                    gridJsObject.setCheckboxChecked(checkbox[0], checked);
                }
            }
        }

        /**
         * Change product position
         *
         * @param {String} event
         */
        function positionChange(event) {
            var element = Event.element(event);

            if (element && element.checkboxElement && element.checkboxElement.checked) {
                categoryProducts.set(element.checkboxElement.value, element.value);
                $('in_category_products').value = Object.toJSON(categoryProducts);
            }
        }

        /**
         * Initialize category product row
         *
         * @param {Object} grid
         * @param {String} row
         */
        function categoryProductRowInit(grid, row) {
            var checkbox = $(row).getElementsByClassName('checkbox')[0];             
        }

        gridJsObject.rowClickCallback = categoryProductRowClick;
        gridJsObject.initRowCallback = categoryProductRowInit;
        gridJsObject.checkboxCheckCallback = registerCategoryProduct;

        if (gridJsObject.rows) {
            gridJsObject.rows.each(function (row) {
                categoryProductRowInit(gridJsObject, row);
            });
        }
    };
});

Then remove position field from Company/Mymodule/Block/Adminhtml/Mymodule/Tab/Product.php