NbSelectComponent closing immediately inside NbTreeGridComponent

### NbSelectComponent closing immediately inside NbTreeGridComponent

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
Embedding an `NbSelectComponent` within a cell of an `NbTreeGridComponent` results in the select's menu closing immediately after it has been opened.

**Expected behavior:**
The select's menu should stay open.

**Steps to reproduce:**
Place an `NbSelectComponent` within a cell of `NbTreeGridComponent`.

**Related code:**
https://stackblitz.com/edit/nb-tree-grid-select


**Workaround**

```html

```

```ts
onSelectClicked(select: NbSelectComponent, event: MouseEvent) {
event.stopPropagation();
select.show();
}
```

**Angular, Nebular**
```
"@angular/core": "^8.2.7"
"@nebular/theme": "^4.3.0"
```