[#79] Windows: Fix errors unconvered by E2E tests

Elwig/Windows/MailWindow.xaml.cs

@@ -155,7 +155,7 @@ namespace Elwig.Windows {
         }
 
         protected override async Task OnRenewContext(AppDbContext ctx) {
-            var season = await ctx.Seasons.Include(s => s.PaymentVariants).Where(s => s.Year == Year).SingleAsync();
+            var season = await ctx.Seasons.Include(s => s.PaymentVariants).Where(s => s.Year == Year).SingleOrDefaultAsync();
             var l = new List<string> {
                 MemberDataSheet.Name
             };