実現したいこと
FlaskとSQLAlchemy、JSONを使って、Webアプリケーションを作成しています。
フォームに入力された文字列をデータベースに格納し、HTMLで書いたページにも表示しようとしています。

困っていること
エラーが出てどのように解決すればいいのか見当がつきません。

エラー文

$ FLASK_APP=app.py FLASK_DEBUG=true flask run
 * Serving Flask app "app.py" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: on
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
 * Restarting with stat
/Users/username/.pyenv/versions/3.6.0/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py:835: FSADeprecationWarning: SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and will be disabled by default in the future.  Set it to True or False to suppress this warning.
  'SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and '
 * Debugger is active!
 * Debugger PIN: 232-439-885
/Users/username/.pyenv/versions/3.6.0/lib/python3.6/site-packages/flask_sqlalchemy/__init__.py:835: FSADeprecationWarning: SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and will be disabled by default in the future.  Set it to True or False to suppress this warning.
  'SQLALCHEMY_TRACK_MODIFICATIONS adds significant overhead and '
127.0.0.1 - - [23/Oct/2019 10:51:00] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [23/Oct/2019 10:51:01] "GET /favicon.ico HTTP/1.1" 404 -
(<class 'AttributeError'>, AttributeError("'ImmutableMultiDict' object has no attribute 'get_json'",), <traceback object at 0x10517fcc8>)
127.0.0.1 - - [23/Oct/2019 10:51:05] "POST /todos/create HTTP/1.1" 400 -

実行しているプログラム
App.py

from flask import Flask, render_template, request, redirect, url_for, abort
from flask_sqlalchemy import SQLAlchemy
import sys

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'postgres://username@localhost:5432/todoappbegin'
db = SQLAlchemy(app)

class Todo(db.Model):
    __tablename__ = 'todos'
    id = db.Column(db.Integer, primary_key=True)
    description = db.Column(db.String(), nullable=False)

    def __repr__(self):
        return f'<Todo {self.id}{self.description}>'

db.create_all()

@app.route('/todos/create', methods=['POST'])
def create_todo():
  error = False
  body = {}
  try:
    description = request.form.get_json()['description']
    todo = Todo(description=description)
    db.session.add(todo)
    db.session.commit()
    body['description'] = todo.description
  except:
    error = True
    db.session.rollback()
    print(sys.exc_info())
  finally:
    db.session.close()
  if error:
    abort (400)
  else:
    return jsonify(body)

@app.route('/')
def index():
    return render_template('index.html', data=Todo.query.all())

Index.html

<html>
<head>
    <title>Todo App</title>
<style>
    .hidden{
        display:  none;
    }
</style>
</head>
<body>
    <form method="post" action="/todos/create">
        <input type= "text" name="description" />
        <input type= "submit" value="Create" />
    </form>
    <div id= "error" class="hidden">Something went wrong!</div>
    <ul>
        {% for d in data %}
        <li>{{d.description}}</li>
        {% endfor %}
    </ul>
    <script>
      const descInput = document.getElementById('description');
      document.getElementById('form').onsubmit = function(e) {
        e.preventDefault();
        const desc = descInput.value;
        descInput.value = '';
        fetch('/todos/create', {
          method: 'POST',
          body: JSON.stringify({
            'description': desc,
          }),
          headers: {
            'Content-Type': 'application/json',
          }
        })
        .then(response => response.json())
        .then(jsonResponse => {
          console.log('response', jsonResponse);
          li = document.createElement('li');
          li.innerText = desc;
          document.getElementById('todos').appendChild(li);
          document.getElementById('error').className = 'hidden';
        })
        .catch(function() {
          document.getElementById('error').className = '';
        })
      }
    </script>
</body>
</html>

試したこと
Postgresで確認したところ、

class Todo(db.Model):
    __tablename__ = 'todos'
    id = db.Column(db.Integer, primary_key=True)
    description = db.Column(db.String(), nullable=False)

    def __repr__(self):
        return f'<Todo {self.id}{self.description}>'

db.create_all()

部分のテーブル作成には成功していることがわかりましたが入力データは入っていないようです。

# \dt
       List of relations
 Schema | Name  | Type  | Owner 
--------+-------+-------+-------
 public | todos | table | username
(1 row)

# select * from todos;
 id | description 
----+-------------
(0 rows)

実行環境
Python 3.6.0
Flask 1.1.1
Sqlalchemy　1.3.10
psql (PostgreSQL) 11.5
ブラウザ Google Chrome