Fix Font NullReferenceException

src/GuiApp/Avalonia/FileTime.GuiApp.Font/Services/FontService.cs

@@ -29,6 +29,6 @@ public class FontService : IFontService
     public string? GetMainFont()
     {
         var installedFonts = FontManager.Current.GetInstalledFontFamilyNames().ToList();
-        return _fontConfiguration.CurrentValue.Main.FirstOrDefault(f => installedFonts.Contains(f));
+        return _fontConfiguration.CurrentValue.Main.Find(installedFonts.Contains);
     }
 }

src/GuiApp/Avalonia/FileTime.GuiApp/ViewModels/MainWindowViewModel.cs

@@ -1,16 +1,12 @@
-using System.Reflection;
+using System.Reactive.Linq;
+using System.Reflection;
 using Avalonia.Input;
 using FileTime.App.Core.Services;
 using FileTime.App.Core.UserCommand;
-using FileTime.App.Core.ViewModels;
 using FileTime.Core.Models;
-using FileTime.Core.Services;
 using FileTime.Core.Timeline;
 using FileTime.GuiApp.Services;
 using FileTime.Providers.Local;
-using InitableService;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
-using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Logging;
 using MvvmGen;
 
@@ -31,7 +27,7 @@ namespace FileTime.GuiApp.ViewModels;
 public partial class MainWindowViewModel : IMainWindowViewModelBase
 {
     public bool Loading => false;
-    public IObservable<string?> MainFont => _fontService.MainFont;
+    public IObservable<string?> MainFont => _fontService.MainFont.Select(x => x ?? "");
     public IGuiAppState AppState => _appState;
     public string Title { get; private set; }