After click on nb-select component occurs scroll collapse on page

### Issue type

**I'm submitting a

* [x ] bug report
* [ ] feature request

### Issue description
There are several nb-select drop-downs on page in nb-layout tag.
nb-layout will auto add style : overflow-y : hidden after click the nb-select component.
But then click on several drop-downs without select any option occurs scroll collapse on page

**Current behavior:**

When click on first drop-down, nb-layout will auto add style : overflow-y : hidden, after click on next nb-select, style : overflow-y : hidden disappear, and when click on next nb-select collapse scroll on page.
**Expected behavior:**

nb-select behavior should be proper
**Steps to reproduce:**

This bug only reproduce if there are several drop-down in vertical position, and direction of clicking starts from lowest drop-down to upper. (direction from down to up, but from up to down isn't reproduce, style : overflow-y : hidden not disappear!)
Same situation if there are several drop-down elements in horizontal position, and traverse direction is from right to left.
**Related code:**

```

### Other information:

**npm, node, OS, Browser**
```

Node, npm: node 12
OS: Windows (7/8/10).
Browser: Chrome

```

**Angular, Nebular**
```
"@angular/core": "^8.0.0",
"@nebular/theme": "^4.6.0",
```