Theme switch support (#3568)

### What problem does this PR solve?
- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

web/src/components/retrieval-documents/index.tsx

@@ -35,7 +35,7 @@ const RetrievalDocuments = ({
             >
               <Space>
                 <span>
-                  {selectedDocumentIds.length ?? 0}/{documents.length}
+                  {selectedDocumentIds?.length ?? 0}/{documents?.length ?? 0}
                 </span>
                 {t('knowledgeDetails.filesSelected')}
               </Space>