Looker not showing html output from embeded Apps Script

[poseidon]

I've a Google Sheet with an Apps Script. That Apps Script takes the index.html and creates a diagram inside a modal dialog.

I can embed the sheet into Looker Studio using Insert -> URL Embed. After do this I have full access to the sheet inside Looker Studio. I can also trigger Apps Script functions. BUT when I'm trying to create the diagram I'm getting the error "docs.google.com has rejected the connection".

I don't understand why this happens and I'd really like to be able to view that diagram inside Looker Studio. Any idea how to solve this?

Apps Script Function

function updateGraph() {
  const ui = SpreadsheetApp.getUi();

  var html = HtmlService.createHtmlOutputFromFile("index")
    .setHeight(1080)
    .setWidth(1500);

  html.setXFrameOptionsMode(HtmlService.XFrameOptionsMode.ALLOWALL);

  const name = "Title of a great diagram";
  ui.showModalDialog(html, name);
}