删除当前用户后如何linkWithCredential？

Question

在我的应用中，我尝试遵循有关如何link multiple auths in Firebase的文档，但是当我按照链接到现有帐户的示例进行操作时， linkWithCredential 失败，错误为当前没有用户登录。

// Get reference to the currently signed-in user
    let prevUser = firebase.auth().currentUser;
    // Sign in user with another account
    firebase.auth().signInWithCredential(_credential).then((user) => {
      let currentUser = user.user;
      // Merge prevUser and currentUser data stored in Firebase.
      // Note: How you handle this is specific to your application

      // After data is migrated delete the duplicate user
      return currentUser.delete().then(() => {
        // Link the OAuth Credential to original account
        console.log("!!DELETE FACEBOOK")
        return prevUser.linkWithCredential(_credential).then(() => {
          console.log("!!LINKING-FACEBOOK")
        }).catch((error) => {
          console.log("!!LINKING ERROR", error)
        });
      }).then(() => {
        console.log("!!SIGNIN-LINKED")
        // Sign in with the newly linked credential
        return firebase.auth().signInWithCredential(_credential)
      });
    }).catch((error) => {
      console.log("Sign In Error", error);
    });

让我感到困惑的是，如何才能重新登录以前的帐户，以便能够将新删除的帐户凭据链接到该帐户？