dateChange event of nb-calendar is emitting more that one date

### Issue type

**I'm submitting a ...** (check one with "x")

* [x ] bug report
* [ ] feature request

### Issue description

**Current behavior:**
I'm using a simple nb-calendar in a dialog, I notice, that if I: Open the calendar and click outside the dialog (this close the dialog and calendar) And I repeat the process, lets say 3 times more. When I select the date it will emit the date 4 times. ( 3 times that I cancel and the time I cancel)

I believe, that is a subscription that keeps opened and should be close, when I select the date, all the subscriptions trigger and that's why I receive 4 emitis

**Expected behavior:**

Emit once

**Steps to reproduce:**
Im using a simple nb-calendar, I notice, that if I: Open the calendar and click outside (this close the calendar) And I repeate the process, lets say 3 times more. When I select the date it will emit the date 4 times. ( 3 times that I cancel and the time I cancel)

**Related code:**

```HTML

```

a service

```Typescript
dateChange(selectedDate: Date) {
this.pickerCalendarAction.changedDate$.next(selectedDate);
}
```
changedDate$ is emitting 4 values ( in the example I gave)
### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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