Message if SearchBar is empty

I can not leave a% void% empty when I click on SearchBar , I have to force the user to fill it. Here is the code:

        //Pesquisa a cidade quando o botão de Pesquisa do teclado é clicado
        private async void MainSearchBar_OnSearchButtonPressed(object sender, EventArgs e)
        {
            {
                await Task.Factory.StartNew(() =>       //Task, classe que facilita o uso de Threads(processamento paralelo). 
                {
                    IsLoading = true;
                    NotifyPropertyChanged("Loading");
                    string cidade;
                    cidade = MainSearchBar.Text;

                    BeginInvokeOnMainThreadAsync(() =>  //conseguirmos fazer a thread ser executada dentro da Thread Principal responsável pela UI
                    {
                        AtualizaDados();

                    }).GetAwaiter().GetResult();

                    IsLoading = false;
                    NotifyPropertyChanged("Loading");
                });
            }
        }

I'm trying to put the following code so that it can work (but is not giving rs):

                /*Se tiver vazio...
                if (MainSearchBar.Text == null)
                {
                    await DisplayAlert("Comunicado", "Campo de pesquisa vazio", "OK");
                }
                else*/

It gives the impression that buscar has some function that by clicking SearchBar and it is empty it simply does nothing.