I cannot see the app widget from my theme editor

While the app is designed to be displayed in your public product pages, it should also work when you are previewing your product pages in your theme editor:

However, some customised or third-party themes may not include the information about the current product in your admin zone and the app widget might not be displayed there. Don't worry, the app will still be visible in your public pages.

Anyway, if you want to fix it, you can include the following code in your product templates:

     <script id="ProductJson-product-template">{{ product | json }}</script>

This is the standard Shopify snippet to include the product information in your product template.