firebase automatic login problem when authentication does not exist

Question

I'm making login function. I want to add automatic login. So, I referenced the code below.

if (currentUser != null) {
    Log.d(TAG,"");
    /* Intent  */
}else{
    Log.d(TAG,"");
}

For testing, I deleted user from firebase authentication console. I expected the automatic login to fail, but it wasn't.

It seems that there is cache memory remaining. If I clear the cache memory, automatic login may not work.

So, I want to add function to know user has authentication to automatic login.

Any other method is fine. I'd appreciate it if you could tell me how to do it.