diff --git a/src/components/block-editor/index.js b/src/components/block-editor/index.js
index 6b236cc8..1be804ed 100644
--- a/src/components/block-editor/index.js
+++ b/src/components/block-editor/index.js
@@ -125,13 +125,21 @@ function BlockEditor( props ) {
 
 	// For back-compat with older iso-editor
 	useEffect( () => {
+		const html = document.querySelector( 'html' );
+
 		if ( isFullscreenActive ) {
 			// @ts-ignore
-			document.querySelector( 'html' ).classList.add( 'is-fullscreen-mode' );
+			html.classList.add( 'is-fullscreen-mode' );
 		} else {
 			// @ts-ignore
-			document.querySelector( 'html' ).classList.remove( 'is-fullscreen-mode' );
+			html.classList.remove( 'is-fullscreen-mode' );
 		}
+
+		return () => {
+			if ( html ) {
+				html.classList.remove( 'is-fullscreen-mode' );
+			}
+		};
 	}, [ isFullscreenActive ] );
 
 	return (