Text in jetpack compose not showing text?

This class simply emit character after 1-sec delay

class MainViewModel: ViewModel() {
    var bouncingName = flow<Char> {
        val text="hemant"
        for (letter in text){
            delay(1000L)
            emit(letter)
        }
    }
}

This main class displays string after concatenating string display on the app

class MainActivity : ComponentActivity() {
    @SuppressLint("CoroutineCreationDuringComposition")
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            CompleteGuideKotlinFlowTheme {
                // A surface container using the 'background' color from the theme
                var letter by remember {
                    mutableStateOf(StringBuilder())
                }
                val viewModel = viewModel<MainViewModel>()
                //  var currentValue=viewModel.countDownValue.collectAsState(initial = 10)
                lifecycleScope.launch {
                    viewModel.bouncingName.collect {
                        letter.append(it)
                        Log.d("TAG", "Character in main $letter")
                    }
                }

                Box(modifier = Modifier.fillMaxSize(),
                    contentAlignment = Alignment.Center) {
                    Text(
                        text = letter.toString(),
                        fontSize = 30.sp
                    )
                }
            }
        }
    }
}

But nothing is showing on the app.
I don't understand why?