MailWindow: Add try/catch block around document creation

Elwig/Windows/MailWindow.xaml.cs

@@ -427,6 +427,7 @@ namespace Elwig.Windows {
             var memberDocs = recipients.Select(m => new {
                 Member = m,
                 Docs = docs.SelectMany<SelectedDoc, GeneratedDoc>(doc => {
+                    try {
                         if (doc.Type == DocType.Custom) {
                             return [new GeneratedDoc((string)doc.Details!)];
                         } else if (doc.Type == DocType.MemberDataSheet) {
@@ -463,6 +464,10 @@ namespace Elwig.Windows {
                         } else {
                             throw new NotImplementedException("Invalid DocType");
                         }
+                    } catch (Exception exc) {
+                        MessageBox.Show(exc.Message, "Fehler", MessageBoxButton.OK, MessageBoxImage.Error);
+                        return [];
+                    }
                 }).ToList()
             }).ToList();