error: The operator '[]' isn't defined for the type 'Object'. of flutter

Question

I'm going to update the data related to Comant in the post documentary, and if there are too many Comant, I'm going to implement writing down how many Comant there are, but there is an error.

int numOfComments = postSnapshot.data[KEY_NUMOFCOMMENTS];
         tx.update(postRef, {
           KEY_NUMOFCOMMENTS: numOfComments+1,
           KEY_LASTCOMMENT: commentData[KEY_LASTCOMMENT],
           KEY_LASTCOMMENTTIME: commentData[KEY_COMMENTTIME],
           KEY_LASTCOMMENTOR: commentData[KEY_USERKEY],

I wrote it like this, but there's an error. I looked it up, and I was told to write () next to the data, but there's an error.

There's an error like this. error: The operator '[]' isn't defined for the type 'Object'. (undefined_operator at [instagram] lib/repo/comment_network_repository.dart:21)enter image description here

Is there a problem with KEY_NUMOFCOMENTS?

const KEY_NUMOFCOMMENTS='num_of_last_comments';