always ensure sessions exists

lib/single-file/frame-tree.js

@@ -29,10 +29,9 @@ this.frameTree = this.frameTree || (() => {
 	const TIMEOUT_INIT_REQUEST_MESSAGE = 500;
 	const TOP_WINDOW = isTopWindow(window);
 
-	let sessions, windowId;
+	let sessions = new Map(), windowId;
 
 	if (TOP_WINDOW) {
-		sessions = new Map();
 		windowId = "0";
 	}
 	addEventListener("message", event => {
@@ -88,7 +87,7 @@ this.frameTree = this.frameTree || (() => {
 	}
 
 	function initResponse(message) {
-		const windowData = top.sessions.get(message.sessionId);
+		const windowData = sessions.get(message.sessionId);
 		if (windowData) {
 			message.framesData.forEach(messageFrameData => {
 				let frameData = windowData.frames.find(frameData => messageFrameData.windowId == frameData.windowId);
@@ -106,7 +105,7 @@ this.frameTree = this.frameTree || (() => {
 			});
 			const remainingFrames = windowData.frames.filter(frameData => !frameData.processed).length;
 			if (!remainingFrames) {
-				top.sessions.delete(message.sessionId);
+				sessions.delete(message.sessionId);
 				windowData.resolve(windowData.frames.sort((frame1, frame2) => frame2.windowId.split(".").length - frame1.windowId.split(".").length));
 			}
 		}