Exception: Cannot call SpreadsheetApp.getUi() from this context. (line 1) - google-apps-script

Question

I have a project with two scripts, that is bound to a Google spreadsheet.
The first script opens the UI
var ui = SpreadsheetApp.getUi();
var userProperties = PropertiesService.getUserProperties();
const TOKEN_OLD = 'api.token_old';
function onOpen(){
ui.createMenu('CREDENZIALI')
.addItem('Imposta TOKEN', 'setToken')
.addToUi();
}
function setToken(){
var scriptValue = ui.prompt('Per favore inserisci il token.' , ui.ButtonSet.OK);
userProperties.setProperty(TOKEN_OLD, scriptValue.getResponseText());
}
And the second one does the business logic and implements a custom function that is meant to be used into the spreadsheet.
function getFromApi(book_id,dato) {...}
Obviously, this API is behind authentication and I don't want to hardcode a token into the script, so I've created the first UI in order to let the end-user manage it at runtime.
The UI works flawlessly but when I try to invoke the custom function, I get this error:
I am the owner of the files. What am I missing ?

Answer 1

Try to add SpreadsheetApp.getUi(); inside onOpen() and setToken() separately.
The following script works as standalone:
var userProperties = PropertiesService.getUserProperties();
const TOKEN_OLD = 'api.token_old';
function onOpen(){
SpreadsheetApp.getUi()
.createMenu('CREDENZIALI')
.addItem('Imposta TOKEN', 'setToken')
.addToUi();
}
function setToken(){
var ui = SpreadsheetApp.getUi();
var scriptValue = ui.prompt('Per favore inserisci il token.' , ui.ButtonSet.OK);
userProperties.setProperty(TOKEN_OLD, scriptValue.getResponseText());
}
However, you can not use a custom formula to call this function. See related articles:
Custom Functions in Google Sheets
Custom function throws a "You do not have the permission required to setValue" error