fixed dispatching of end stage event

lib/single-file/single-file-core.js

@@ -220,8 +220,9 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 					log("  -- ENDED   task =", task.action, "delay =", Date.now() - startTime);
 				}
 			});
+			let parallelTasksPromise;
 			if (STAGES[step].parallel) {
-				return await Promise.all(STAGES[step].parallel.map(task => {
+				parallelTasksPromise = await Promise.all(STAGES[step].parallel.map(task => {
 					let startTime;
 					if (DEBUG) {
 						startTime = Date.now();
@@ -235,11 +236,14 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 					}
 					return promise;
 				}));
+			} else {
+				parallelTasksPromise = Promise.resolve();
 			}
 			this.onprogress(new ProgressEvent(STAGE_ENDED, { pageURL: this.options.url, step, frame }));
 			if (DEBUG) {
 				log("**** ENDED   STAGE", step, "****");
 			}
+			return parallelTasksPromise;
 		}
 
 		executeTask(task) {