flask invoke a function and return the output

The purpose is to in the UI select a script and upload a file, and then return the output, now I'm using a dummy script to link up the front and back end.

My folder structure

|-app
   |-main
      |- ...
      |-views.py  <---- this!
   |-script_mgmt
      |-__init__.py
      |-manager.py  <---- this!
      |-scripts
         |-__init__.py
         |-sample.py  <---- and this!
|-...
|-run.py

This is the snippet in view.py

...
@main.route('/', methods = ['GET', 'POST'])
def index():
    form = UploadForm()
    task = None
    file = None
    if form.validate_on_submit():
        # task = form.task.data
        file = form.file.data
        mger = Manager()
        task = 'sample'
        output = mger.invoke(task, file)
        return output
    return render_template('index.html', form = form)

And manager.py

class Manager():

    @staticmethod
    def invoke(script, file):
        from importlib import import_module
        module = import_module(f'app.script_mgmt.scripts.{script}')
        return module.main(file)

And sample.py

def main(file):
    sample(file)

def sample(file):
    return 'Sample script triggered'

I expected the output to be "Sample script triggered" which would be rendered as plain text in my browser, but I somehow got this error saying it returned None.

TypeError: The view function did not return a valid response. The function either returned None or ended without a return statement.

See below for full error messages.

Traceback (most recent call last)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 2464, in __call__
return self.wsgi_app(environ, start_response)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 2450, in wsgi_app
response = self.handle_exception(e)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 1867, in handle_exception
reraise(exc_type, exc_value, tb)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 1953, in full_dispatch_request
return self.finalize_request(rv)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 1968, in finalize_request
response = self.make_response(rv)
File "C:\Users\sliu\Documents\GitHub\script-bank\venv\lib\site-packages\flask\app.py", line 2098, in make_response
"The view function did not return a valid response. The"
TypeError: The view function did not return a valid response. The function either returned None or ended without a return statement.