ViewModel.cs

private readonly Task _initializingTask;

public ViewModel()
{
    _initializingTask = Init();
}

private async Task Init()
{
    var file = await Package.Current.InstalledLocation.GetFileAsync(@"Assets\Text.txt");
    Text = await FileIO.ReadTextAsync(file);
}

private async void OnSomeCommand()
{
    // check if initialization is complete
    if (_initializingTask.IsCompleted)
    {
        // do some action
    }

    // or even await the initialization to complete if it hasn't already
    await _initializingTask;
}