fix(NextTickAction): fix unsubscription behavior

- fix clearImmediate behavior prevents nextTickScheduler throws if it's
being used multiple time

closes #582

src/schedulers/NextTickAction.ts

@@ -4,8 +4,7 @@ import ImmediateAction from './ImmediateAction';
 import Action from './Action';
 
 export default class NextTickAction<T> extends ImmediateAction<T> {
-
- id: number;
+ private id: any;
 
  schedule(state?: any): Action {
  if (this.isUnsubscribed) {
@@ -21,7 +20,7 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
  if (!scheduler.scheduled) {
  scheduler.scheduled = true;
  this.id = Immediate.setImmediate(() => {
- this.id = void 0;
+ this.id = null;
  this.scheduler.scheduled = false;
  this.scheduler.flush();
  });
@@ -30,8 +29,7 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
  return this;
  }
 
- unsubscribe() {
-
+ unsubscribe(): void {
  const id = this.id;
  const scheduler = this.scheduler;
 
@@ -40,10 +38,11 @@ export default class NextTickAction<T> extends ImmediateAction<T> {
  if (scheduler.actions.length === 0) {
  scheduler.active = false;
  scheduler.scheduled = false;
- if (id) {
- this.id = void 0;
- Immediate.clearImmediate(id);
- }
+ }
+
+ if (id) {
+ this.id = null;
+ Immediate.clearImmediate(id);
  }
  }
 }