I'm not sure exactly why, but after merging your code with mine it suddenly started working perfectly.

Thank you.

The error Cannot read properties of undefined (reading 'forEach') suggests that somewhere in your code, you are trying to call forEach on an undefined value. However, you mentioned that there is no forEach in your code, which indicates the issue might be coming from the data structure or response you are handling.

Let's go through the code to identify potential issues and ensure everything is correctly handled.

1. Initialization of this.rpc: Make sure this.rpc is properly initialized.
2. Handling the response from _fetchData: Ensure the data returned by _fetchData is what you expect it to be.
3. Error Handling: Add error handling to catch any issues when fetching or processing data.

Here is the revised version of your code with added error handling and some corrections:

 
/** @odoo-module **/

import publicWidget from "@web/legacy/js/public/public_widget";

publicWidget.registry.BannerManager = publicWidget.Widget.extend({

    selector: '.banner-manager',

    init: function () {

        this._super(...arguments);

        this.rpc = this.bindService("rpc");

    },

    start: async function () {

        let bannerRow = this.el.querySelector('#banner-manager-row');

        if (bannerRow) {

            let html = '';

            let count = 0;

            try {

                const banners = await this._fetchData();

                banners.forEach(banner => {

                    if (count % 2 === 0) {

                        html += `
                                    
                                 
`;

                    } else {

                        html += `
                                    
                                 
`;

                    }

                    count++;

                });

                bannerRow.innerHTML = html;

            } catch (error) {

                console.error('Error fetching banners:', error);

            }

        }

    },

    async _fetchData() {

        try {

            const bannerData = await this.rpc('/get_banners', {});

            if (!Array.isArray(bannerData)) {

                throw new Error('Invalid banner data format');

            }

            return bannerData;

        } catch (error) {

            console.error('Error in _fetchData:', error);

            throw error;

        }

    },

});

export default publicWidget.registry.BannerManager;