当我尝试连接 google firebase 实时数据库时，出现此错误：
ValueError: The default Firebase app already exists. This means you called
initialize_app() more than once without providing an app name as the second argument. In
most cases you only need to call initialize_app() once. But if you do want to initialize
multiple apps, pass a second argument to initialize_app() to give each app a unique name.
这是我的代码：
import firebase_admin
from firebase_admin import credentials
from firebase_admin import db
cred = credentials.Certificate('firebase-sdk.json')
firebase_admin.initialize_app(cred, {
'databaseURL': 'https://test-139a6-default-rtdb.firebaseio.com/'
})