MainActivity.kt

public override fun onStart() {
        super.onStart()
        // Check if user is signed in (non-null) and update UI accordingly.
        val currentUser = auth.currentUser
        if (currentUser != null) {
            Toast.makeText(
                baseContext, "Already Logged In.",
                Toast.LENGTH_SHORT
            ).show()
            startHomeActivity()
        }
    }