Drop-down Menus (NbContextMenu & NbSelect) do not work after login without refreshing the app first.

### Issue type

**I'm submitting a bug report**

### Issue description

**Current behavior:**

I should start out by saying that I'm creating a MEAN Stack Application that has **user login and registration** using Nebular components and the **email NbPasswordAuthStrategy**. To this point I was able to create a basic application with a sidebar, a header and some users. **The problem appears when a user signs-in to the application and attempts to click on the NbSelect component that appears on the top-right corner of the application**. The odd thing about this is that the problem goes away if you refresh the page after signing-in:

Here is the that error appears in the DevTools console up-close:
```
ERROR TypeError: Cannot read property 'appendChild' of null
at NbOverlayContainerAdapter._createContainer (index.js:2006)
at NbOverlayContainerAdapter.getContainerElement (overlay.js:1198)
at NbOverlay._createHostElement (overlay.js:4233)
at NbOverlay.create (overlay.js:4192)
at NbOverlayService.create (index.js:2572)
at NbDynamicOverlay.createOverlay (index.js:12052)
at NbDynamicOverlay.show (index.js:12014)
at SafeSubscriber._next (index.js:12243)
at SafeSubscriber.__tryOrUnsub (Subscriber.js:183)
at SafeSubscriber.next (Subscriber.js:122)
```

**Note1**: I did not use the ngx-admin-starter-kit, but rather I just created my own Angular App from scratch.
**Note2**: While this example demonstrates the error with NbContextMenu, I assure you the exact same error appears under the same circumstances with the NbSelectComponent.

**Expected behavior:**

The appropriate would of course be that the drop-down context menu would appear instead of giving off this error.

**Steps to reproduce:**

Load up the site -> Create a user -> Login -> Click on the user NbContextMenu at the top right corner without refreshing the page.

**Related code:**

I've created [this public repository](https://github.com/Nikitas-io/Ngx-Dropdown-Issue) so that reviewers can look at the issue thoroughly and I also [deployed the project to Heroku for a live preview](https://nebularmeanstack.herokuapp.com/#/). You can create a new user to try it out yourself.

The offending code that refers to the Context Menu is located in the **app.component.ts** file:
```
export class AppComponent implements OnInit {

// User login flag.
isLoggedIn = false;
// A user variable, which will store the JTW token payload inside of the component.
user = {};
// NbContextMenu dropdown options.
dropdownItems = [
{
title: 'Profile',
link: '/profile'
},
{
title: 'Log out',
link: '/auth/logout'
}
];
// The route paths of the application.
items: NbMenuItem[] = [
{
title: 'Home',
link: '/'
},
{
title: 'Users',
link: '/users'
},
{
title: 'Products',
link: '/products'
},
{
title: 'Orders',
link: '/orders'
},
];

constructor(@Inject(NB_WINDOW) private window,
private sidebarService: NbSidebarService,
private authService: NbAuthService,
private usersService: UsersService,
private nbMenuService: NbMenuService,
) { }

ngOnInit() {
// Get the user token.
this.authService.onTokenChange().subscribe((token: NbAuthJWTToken) => {
// Validate the user token.
if (token.isValid()) {
// Receive the token payload and assign it to the `user` variable.
this.user = token.getPayload();
console.log(this.user["_id"]);

// Change the login button to the user on the header.
this.isLoggedIn = true;

// Fetch the signed-in user's data.
this.usersService.getUser(this.user["_id"]).subscribe(user => {
// Set the signed in user's name.
this.user["fullName"] = user["fullName"];
})
}

if (!token.isValid()) {
// Change the User to the Login button on the header.
this.isLoggedIn = false;
}
});

// Context Menu Event Handler.
this.nbMenuService.onItemClick().pipe(
filter(({ tag }) => tag === 'my-context-menu'),
map(({ item: { title } }) => title),
).subscribe((title) => {
// Check if the Logout menu item was clicked.
if (title == 'Log out') {
// Logout the user.
this.authService.logout('email').subscribe((Observable) => {
// Clear the user. (THIS doesn't seem like the right way to do it).
this.user = {};
})
}
});
}

toggle() {
this.sidebarService.toggle(false, 'left');
}

}
```
and its corresponding **app.component.html** file:
```

☰


Login

```
### Other information:
- Node Version: `v12.18.0`
- Npm Version:
```
{
'nebular-app': '1.0.0',
npm: '6.14.5',
ares: '1.16.0',
brotli: '1.0.7',
cldr: '37.0',
http_parser: '2.9.3',
icu: '67.1',
llhttp: '2.0.4',
modules: '72',
napi: '6',
nghttp2: '1.41.0',
node: '12.18.0',
openssl: '1.1.1g',
tz: '2019c',
unicode: '13.0',
uv: '1.37.0',
v8: '7.8.279.23-node.37',
zlib: '1.2.11'
}
```
OS: Windows 10
Browser: Brave Browser (It is a Chrome clone)

**Angular, Nebular**
package.json:
```
{
"name": "nebular-app",
"version": "1.0.0",
"description": "An app constructed with Nebular components",
"main": "server.js",
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
"debug": "node --inspect-brk server.js",
"start": "nodemon server.js"
},
"author": "",
"license": "ISC",
"dependencies": {
"@hapi/joi": "^17.1.1",
"bcryptjs": "^2.4.3",
"body-parser": "^1.19.0",
"dotenv": "^8.2.0",
"express": "^4.17.1",
"jsonwebtoken": "^8.5.1",
"mongojs": "^3.1.0",
"mongoose": "^5.9.25"
}
}
```