check null at app service

modules/cms-kit/src/Volo.CmsKit.Public.Application/Volo/CmsKit/Public/Pages/PageAppService.cs

@@ -15,6 +15,11 @@ namespace Volo.CmsKit.Public.Pages
         public virtual async Task<PageDto> FindByUrlAsync(string url)
         {
             var page = await PageRepository.FindByUrlAsync(url);
+
+            if (page == null)
+            {
+                return null;
+            }
             
             return ObjectMapper.Map<Page, PageDto>(page);
         }