I do not load the text of the OnFileActive in the RichEditBox when reading it C # UWP

I am using the OnFileActive for when I open a file with the "Open With and I choose my program" because it opens and its text is loaded in the RichEditBox and its path appears in the title of the Notepad-style app, then everything goes well since with the function that I put in the MainPage the path is put in the title, I read it and load it in the RichEditBox use the same code that I use with a button that I put with a File Picker that loads it also, but for some reason it does not load it to the RichEditBox. I tried instead of loading the RichEditBox I put it in a MessageDialog and there if they showed it then I do not understand why the RichEditBox does not load it. The code that will show will be OnNavigatedTo in App.xalm.cs and the function in Main.xalm.cs.

async void OnNavigatedTo(object sender, NavigationEventArgs e)
    {
        var args = e.Parameter as Windows.ApplicationModel.Activation.IActivatedEventArgs;
        if (args != null)
        {
            if (args.Kind == Windows.ApplicationModel.Activation.ActivationKind.File)
            {
                var fileArgs = args as Windows.ApplicationModel.Activation.FileActivatedEventArgs;
                string strFilePath = fileArgs.Files[0].Path;
                var file = (StorageFile)fileArgs.Files[0];
                await new MainPage().LoadTextFile(file);
            }
        }    
    }

And the function in the Main.xalm.cs

public async Task LoadTextFile(StorageFile file)
    {
        string text = await FileIO.ReadTextAsync(file);
            rbText.Document.SetText(TextSetOptions.None, text);
            current_path = file.Path;
            current_file = file.Name;
            var appView = Windows.UI.ViewManagement.ApplicationView.GetForCurrentView();
            appView.Title = current_path;
    }