make sure old values exist

lib/single-file/processors/hooks/content/content-hooks-frames-web.js

@@ -224,10 +224,14 @@
 
 	function resetScreenSize() {
 		const scrollingElement = document.scrollingElement || document.documentElement;
-		window.innerHeight = window._singleFile_innerHeight;
-		window.innerWidth = window._singleFile_innerWidth;
-		delete window._singleFile_innerHeight;
-		delete window._singleFile_innerWidth;
+		if (window._singleFile_innerHeight != null) {
+			window.innerHeight = window._singleFile_innerHeight;
+			delete window._singleFile_innerHeight;
+		}
+		if (window._singleFile_innerWidth != null) {
+			window.innerWidth = window._singleFile_innerWidth;
+			delete window._singleFile_innerWidth;
+		}
 		delete scrollingElement.clientHeight;
 		delete scrollingElement.clientWidth;
 		delete screen.height;