signInWithCredential for Facebook Auth in Android didn't pass

Question

got a problem with built-in function signInWithCredential. I have no idea what's wrong, trying most thing from Stack and different sites... If I change "?" to "!!" after mAuth I got error:

java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=64206, result=-1, data=Intent { (has extras) }} to activity (...) java.lang.NullPointerException

Here is the handleFacebookAcessToken function

private fun handleFacebookAccesToken(accessToken: AccessToken?) {
    val progressDialog = ProgressDialog(this@MainActivity)
    progressDialog.setTitle("Sign Up with Facebook")
    progressDialog.setMessage("Please wait, this may take a while...")
    progressDialog.setCanceledOnTouchOutside(false)
    progressDialog.show()

    val credential = FacebookAuthProvider.getCredential(accessToken!!.token)
    val mAuth: FirebaseAuth? = null
    mAuth?.signInWithCredential(credential)
        ?.addOnCompleteListener(this) { task ->
            if (task.isSuccessful)
            {
                val intent = Intent(this, Activity2::class.java)
                startActivity(intent)
                progressDialog.dismiss()
                val user: FirebaseUser = FirebaseAuth.getInstance().currentUser!!
                //updateUI(user)
            }
            else
            {
                val message = task.exception!!.toString()
                Toast.makeText(this, "Error: $message", Toast.LENGTH_LONG).show()
                mAuth?.signOut()
                progressDialog.dismiss()
            }
        }
}

Also the two more functions which can be wrong

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
    super.onActivityResult(requestCode, resultCode, data)
    callbackManager?.onActivityResult(requestCode, resultCode, data)
}

private fun SignInWithFacebook()
{
    if (BuildConfig.DEBUG) {
        FacebookSdk.setIsDebugEnabled(true)
        FacebookSdk.addLoggingBehavior(LoggingBehavior.INCLUDE_ACCESS_TOKENS)
    }
    LoginManager.getInstance().logInWithReadPermissions(this, Arrays.asList("public_profile", "email"))
    LoginManager.getInstance().registerCallback(callbackManager,
        object : FacebookCallback<LoginResult>
        {
            override fun onSuccess(result: LoginResult?) {
                handleFacebookAccesToken(result!!.accessToken)
            }

            override fun onCancel() {

            }

            override fun onError(error: FacebookException?) {
                Toast.makeText(applicationContext, "Error!", Toast.LENGTH_LONG).show()
            }

        })
}

Sorry if I asked something wrong, it's my first post