默认Firebase应用不存在请确保调用initialize_app()初始化SDK

调用initialize_app()初始化SDK。示例代码如下：

import firebase_admin
from firebase_admin import credentials
from firebase_admin import firestore

# Use a service account
cred = credentials.Certificate('path/to/serviceAccountKey.json')
firebase_admin.initialize_app(cred)

# Get a Firestore client
db = firestore.client()

注：上述示例代码使用Python Firebase Admin SDK。具体操作可根据实际情况进行调整。