google.script.run.withSuccessHandler(…).withFailureHandler(…) is not a function

1. I’m integrating my Google Apps Script library into another guest Google Apps Script.

2. Both the library and the guest script are tied-to Google Sheets.

3. The library has a sidebar.html file, which is called into the guest script like this:

   Guest Script > Script.gs

   function onOpen() {
     SpreadsheetApp.getUi()
         .createMenu('My Menu')
         .addItem('Open Sidebar', 'showSidebar')
         .addToUi();
   }
   
   function showSidebar() {
       myLibrary.showSidebar();
   }
   

   This works and creates the UI menu, and clicking it opens the sidebar.html from the library, which runs as an HTML file with all front-end JavaScript <script>s inside of it.

4. In the sidebar.html file (in the library) I have a JavaScript function to communicate with a server-side function through google.script.run, like this:

   Library > sidebar.html

   <script>
       google.script.run
           .withSuccessHandler((backendResponse) => {
               console.log('Successfully got backend response!');
           })
           .withFailureHandler((error) => {
               console.log('Error getting backend response!');
           })
           .myBackendFunctionName(someArgument);
   </script>
   
   

This works alright when I run it in a Google Apps Script directly, but, when I run it from an integrated library, it throws the following error to the guest’s console:

Uncaught TypeError:
google.script.run.withSuccessHandler(…).withFailureHandler(…).myBackendFunctionName
is not a function

What am I doing wrong? and how can I fix that?