Добавил проверку на null и применение темы по умолчанию для диалогов сохранения и открытия файлов.

Author: dosymep

src/dosymep.Xpf.Core/SimpleServices/DxCustomServices/DXCustomDialogs/CustomDXOpenFileDialog.cs

@@ -24,7 +24,14 @@ protected override IDialogHost CreateDialogHost(IntPtr hwndOwner) {
                 ShowInTaskbar = false
             };
 
-            _themeUpdaterService().SetTheme(_theme().HostTheme, fileDialogWindow);
+            IUIThemeService themeService = _theme();
+            IUIThemeUpdaterService themeUpdaterService = _themeUpdaterService();
+
+            if(themeService is null || themeUpdaterService is null) {
+                ThemeManager.SetTheme(fileDialogWindow, Theme.Win10Light);
+            } else {
+                themeUpdaterService.SetTheme(themeService.HostTheme, fileDialogWindow);
+            }
 
             return fileDialogWindow;
         }

src/dosymep.Xpf.Core/SimpleServices/DxCustomServices/DXCustomDialogs/CustomDXSaveFileDialog.cs

@@ -23,9 +23,16 @@ protected override IDialogHost CreateDialogHost(IntPtr hwndOwner) {
                 WindowStyle = WindowStyle.ToolWindow,
                 ShowInTaskbar = false
             };
+           
+            IUIThemeService themeService = _theme();
+            IUIThemeUpdaterService themeUpdaterService = _themeUpdaterService();
 
-            _themeUpdaterService().SetTheme(_theme().HostTheme, fileDialogWindow);
-
+            if(themeService is null || themeUpdaterService is null) {
+                ThemeManager.SetTheme(fileDialogWindow, Theme.Win10Light);
+            } else {
+                themeUpdaterService.SetTheme(themeService.HostTheme, fileDialogWindow);
+            }
+            
             return fileDialogWindow;
         }
     }