Flask PyMongo find_one() return None even when data exists

I just started using MongoDb and I am stuck when using find_one(). The find() function works perfectly but None type is returned when using find_one(). I am stuck at this point. Below is the code that I am using.

#mongo.py
@app.route('/update_user/<emp_id>', methods=['GET', 'POST'])
def update_user(emp_id):
    print("Employee ID:", emp_id)
    output = list(emp.find_one({'emp_id': emp_id}))
    print("Output:", output)
    if request.method == 'POST':
        if not request.form.get['emp_id'] or not request.form.get['first_name'] or not request.form.get['last_name'] \
                or not request.form.get['dept_name'] or not request.form.get['gender'] or not request.form.get['dob'] \
                or not request.form.get['salary'] or not request.form['country_code'] \
                or not request.form.get['mobile_no']:
            flash('Please enter all the fields', 'error')
        else:
            emp_id = int(request.form.get('emp_id'))
            first_name = request.form.get('first_name')
            last_name = request.form.get('last_name')
            dept_name = request.form.get('dept_name')
            gender = request.form.get('gender')
            dob = request.form.get('dob')
            salary = float(request.form.get('salary'))
            country_code = int(request.form['country_code'])
            mobile_no = int(request.form.get('mobile_no'))
            emp.update_one({"emp_id": emp_id, "first_name": first_name, "last_name": last_name,
                            "dept_name": dept_name, "gender": gender, "dob": dob, "salary": salary,
                            "country_code": country_code, "mobile_no": mobile_no})
            flash('Employee Updated')
            return redirect('user.html')
    return render_template('update_user.html', emp_id=emp_id, output=output)

It throws the below error:

File "D:\projects\EmpMgmtFlask\mongo.py", line 37, in update_user
output = list(emp.find_one({'emp_id': emp_id}))
TypeError: 'NoneType' object is not iterable

find_one() doesn't return a cursor; it returns a single dictionary; so you don't need to wrap it in a list function.

output = emp.find_one({'emp_id': emp_id})