init(form_id) {
        const form = document.forms[form_id];
        if (form === undefined) {
            throw new Error(`Form with ID ${form_id} not found in DOM!`);
        }
        if (this._collection[form_id] !== undefined) {
            throw new Error(`Form with ID ${form_id} already initiated!`);
        }
        this._collection[form_id] = new BunnyFormData(form);
        this._attachChangeAndDefaultFileEvent(form_id);
        this._attachRadioListChangeEvent(form_id);
        this._attachDOMChangeEvent(form_id);
    }
Init form
Must be called after DOMContentLoaded (ready)

@param {string} form_id

@throws Error