Bugged scrollbar when removing windowMode of nb-layout

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

* [x] bug report
* [ ] feature request
* [ ] question about the decisions made in the repository

### Issue description

**Current behavior:**

I've removed windowMode from my layout tag expecting to use the full layout in bigger resolutions, but what i achive was a bugged non-styled scrollbar in front of my application.

**Expected behavior:**

Be able to use nb-layout, removing windowMode without bugging scrollbar.

**Steps to reproduce:**

Clone the ngx-admin project, install the modules and remove windowMode from your choosen layout tag

**Related code:**

```
template: `

`,
```

### Other information:

**npm, node, OS, Browser**
```
Node, v14.17.1 NPM, 7.18.1
OS: Windows 10x64
Browser: Chrome/Firefox
```
**Angular, Nebular**
```
Angular 11, Nebular 7
```