fix: TypeError: Cannot read properties of undefined (reading 'viewport'

infiniflow#1761 (infiniflow#1784)

### What problem does this PR solve?

fix: TypeError: Cannot read properties of undefined (reading 'viewport'
infiniflow#1761

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/pages/add-knowledge/components/knowledge-chunk/components/document-preview/preview.tsx

@@ -41,7 +41,7 @@ const Preview = ({ highlights: state, setWidthAndHeight }: IProps) => {
 
   useEffect(() => {
     if (state.length > 0) {
-      ref.current(state[0]);
+      ref?.current(state[0]);
     }
   }, [state]);