Gutenberg: Dynamic Block - Show saved data in the editor

Im trying to create a Gutenberg dynamic block with dropdown list. I have completed block creation and rendering the block with selected dropdown value in the frontend.

How to set the dropdown selected with previous value when editing a post?

I have tried to access attribute value using props.attributes , but getting undefined

block.js

const { __ } = wp.i18n;
const { registerBlockType, RichText } = wp.blocks;

registerBlockType( 'gut/new-block', {
    title: __( 'new-block - Test' ),
    category: 'common',
    attributes: {
        category_id: {
            type: 'number'
        }
    },
    edit: function( props ) {
        const { attributes: { category_id }, setAttributes } = props;

        function setCategory( event ) {
            const selected = event.target.querySelector( 'option:checked' );
            setAttributes( { category_id: selected.value } );
            event.preventDefault();
        }

        return (
            <div className={ props.className }>                            
                <select value={ category_id } onChange={ setCategory }>
                    <option value="120">Animals</option>
                    <option value="350">Architecture</option>
                    <option value="700">Nature</option>
                    <option value="800">People</option>
                    <option value="432">Tech</option>
                </select>                
            </div>
        );
    },
    save ( props ) {
        return null
    },
});

php

function gut_my_new_block() {
    wp_register_script(
        'gut-my-new-block',
        plugins_url( 'new-block/dist/blocks.build.js', __FILE__ ),
        array( 'wp-blocks', 'wp-element' )
    );

    register_block_type( 'gut/new-block', array(
        'render_callback' => 'gut_render_block_my_newblock',
        'editor_script' => 'gut-my-new-block',
    ) );
}

add_action( 'init', 'gut_my_new_block' );

function gut_render_block_my_newblock($params) {  
    return '<h3>selected category '.$params['category_id'].'</h3>';
}

Select values are saved as strings, so if you change the type to "string" it will work. Or you could parseInt() or Number() the value before saving and then toString() it back when pulling the value.