如何在UWP项目中使用MVVM避免空值

我想添加新的咖啡细节，当我添加使用Addcommand(自定义命令)，但当添加新的细节是null，那么我如何避免这个问题?

模型等级Coffee

public class Coffee : BindableBase
{
    private int coffeeId;
    private string coffeeName;
    public int CoffeeId
    {
        get
        {
            return coffeeId;
        }
        set
        {
            coffeeId = value;
            RaisePropertyChanged("CoffeeId");
        }
    }
    public string CoffeeName
    {
        get
        {
            return coffeeName;
        }
        set
        {
            coffeeName = value;
            RaisePropertyChanged("CoffeeName");
        }
    }
}

查看CoffeeAdd.xaml

<StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="Coffee Id"/>
            <TextBox Width="120" Height="30" Margin="50 0 0 0" Text="{Binding CoffeeId}"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="0 20 0 0">
            <TextBlock Text="Coffee Name"/>
            <TextBox Width="120" Height="30" Margin="20 0 0 0" Text="{Binding CoffeeName}"/>
        </StackPanel>
        <StackPanel Orientation="Horizontal" Margin="0 20 0 0">
            <Button Content="Add" Width="120" Height="30" Command="{Binding AddCommand}"/>
            <Button Content="View" Width="120" Height="30" Margin="20 0 0 0"/>
        </StackPanel>
    </StackPanel>

ViewModel CoffeeAddViewModel

public class CoffeeAddViewModel:BindableBase
{
    private ICoffeeDataService coffeedataservice;
    public CoffeeAddViewModel(ICoffeeDataService dataservice)
    {
        coffeedataservice = dataservice;
        LoadCommand();
    }
    private int _coffeeId;
    private string _coffeeName;
    public int CoffeeId
    {
        get
        {
            return _coffeeId;
        }
        set
        {
            _coffeeId = value;
            RaisePropertyChanged("CoffeeId");
        }
    }
    public string CoffeeName
    {
        get
        {
            return _coffeeName;
        }
        set
        {
            _coffeeName = value;
            RaisePropertyChanged("CoffeeName");
        }
    }
    public ICommand AddCommand { get; set; }
    private void LoadCommand()
    {
        AddCommand = new CustomCommand(add, canadd);
    }
    private async void add(object obj)
    {
        coffeedataservice.AddCoffee(new Model.Coffee { CoffeeId = _coffeeId, CoffeeName = _coffeeName });
        var dialog = new MessageDialog("Successfully Added");
        await dialog.ShowAsync();
    }
    private bool canadd(object obj)
    {
        return true;
    }
}