diff --git a/apps/red-ui/src/app/modules/file-preview/file-preview-screen.component.ts b/apps/red-ui/src/app/modules/file-preview/file-preview-screen.component.ts
index dadb3656e..565b97eec 100644
--- a/apps/red-ui/src/app/modules/file-preview/file-preview-screen.component.ts
+++ b/apps/red-ui/src/app/modules/file-preview/file-preview-screen.component.ts
@@ -556,9 +556,9 @@ export class FilePreviewScreenComponent extends AutoUnsubscribe implements OnIni
         const pageContainer = iframeDoc.getElementById(elementId);
         if (pageContainer) {
             if (file.excludedPages.includes(currentPage)) {
-                pageContainer.classList.add('excluded-page');
+                pageContainer?.classList?.add('excluded-page');
             } else {
-                pageContainer.classList.remove('excluded-page');
+                pageContainer?.classList?.remove('excluded-page');
             }
         }
     }
@@ -793,13 +793,13 @@ export class FilePreviewScreenComponent extends AutoUnsubscribe implements OnIni
     #updateViewerPosition() {
         if (this.isDocumine) {
             if (this._documentInfoService.shown()) {
-                document.getElementById('viewer').classList.add('document-info');
+                document.getElementById('viewer')?.classList?.add('document-info');
             } else {
-                document.getElementById('viewer').classList.remove('document-info');
+                document.getElementById('viewer')?.classList?.remove('document-info');
             }
-            document.getElementById('viewer').classList.add('documine-viewer');
+            document.getElementById('viewer')?.classList?.add('documine-viewer');
             return;
         }
-        document.getElementById('viewer').classList.add('redaction-viewer');
+        document.getElementById('viewer')?.classList?.add('redaction-viewer');
     }
 }