input: Write a function that creates a Flask application to submit form data and store it in a SQLite database. The form should have elements for Name, Email, and Phone. The function should return the Flask application object.
  assert create_app()
output: 
  def create_app():
      from flask import Flask, request, render_template_string
      import sqlite3
  
      app = Flask(__name__)
  
      conn = sqlite3.connect('form_data.db')
      cursor = conn.cursor()
      cursor.execute('''
          CREATE TABLE IF NOT EXISTS submissions (
              id INTEGER PRIMARY KEY AUTOINCREMENT,
              name TEXT NOT NULL,
              email TEXT NOT NULL,
              phone TEXT NOT NULL
          )
      ''')
      conn.commit()
      conn.close()
  
      @app.route('/', methods=['GET', 'POST'])
      def index():
          if request.method == 'POST':
              name = request.form['name']
              email = request.form['email']
              phone = request.form['phone']
  
              conn = sqlite3.connect('form_data.db')
              cursor = conn.cursor()
              cursor.execute('''
                  INSERT INTO submissions (name, email, phone)
                  VALUES (?, ?, ?)
              ''', (name, email, phone))
              conn.commit()
              conn.close()
  
              return 'Form submitted successfully!'
  
          return render_template_string('''
              <form method="post">
                  <label for="name">Name:</label>
                  <input type="text" id="name" name="name" required><br>
                  <label for="email">Email:</label>
                  <input type="email" id="email" name="email" required><br>
                  <label for="phone">Phone:</label>
                  <input type="text" id="phone" name="phone" required><br>
                  <input type="submit" value="Submit">
              </form>
          ''')
  
      return app
  