Bottle App gives Internal Server Error when accessing default page

I am just getting started with Bottle. I have a sample app on GitHub. The main module app.py (in the Application folder) looks like below

"""
This script runs the application using a development server.
"""

import bottle
import os
import sys

# routes contains the HTTP handlers for our server and must be imported.
import routes

if '--debug' in sys.argv[1:] or 'SERVER_DEBUG' in os.environ:
    # Debug mode will enable more verbose output in the console window.
    # It must be set at the beginning of the script.
    bottle.debug(True)

def wsgi_app():
    """Returns the application to make available through wfastcgi. This is used
    when the site is published to Microsoft Azure."""
    return bottle.default_app()

if __name__ == '__main__':
    PROJECT_ROOT = os.path.abspath(os.path.dirname(__file__))
    STATIC_ROOT = os.path.join(PROJECT_ROOT, 'static').replace('\\', '/')
    HOST = os.environ.get('SERVER_HOST', 'localhost')
    try:
        PORT = int(os.environ.get('SERVER_PORT', '5555'))
    except ValueError:
        PORT = 5555

    @bottle.route('/static/<filepath:path>')
    def server_static(filepath):
        """Handler for static files, used with the development server.
        When running under a production server such as IIS or Apache,
        the server should be configured to serve the static files."""
        return bottle.static_file(filepath, root=STATIC_ROOT)

    # Starts a local test server.
    bottle.run(server='wsgiref', host=HOST, port=PORT)

The requirements.txt file has

bottle
gunicorn

as dependencies. I am using Python 3.7.2. After running pip install -r requirements.txt,

I ran python app.py. The server starts up and I can access the default page without error error

I tried running the server using gunicorn as below

gunicorn -w 2 -b 0.0.0.0:8080 app:wsgi_app    

The server starts up fine, but when I access the default page, I get

Traceback (most recent call last):
  File "/Users/<user>/Codebase/test-bottle/venv/lib/python3.8/site-packages/gunicorn/workers/sync.py", line 134, in handle
    self.handle_request(listener, req, client, addr)
  File "/Users/<user>/Codebase/test-bottle/venv/lib/python3.8/site-packages/gunicorn/workers/sync.py", line 175, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
TypeError: wsgi_app() takes 0 positional arguments but 2 were given

Please let me know what I've done wrong.

from Bottle App gives Internal Server Error when accessing default page