nb-select / nb-option throws error in console

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

I'm trying to show the values from nb-option, but when i click to expand, the console throws an error:

- Console Error:

- Nebular components:
- _Note that is selected but not show the others options!_

**Expected behavior:**

Expand the nb-select and show the options from nb-option

**Steps to reproduce:**

Using Bulma CSS + Nebular Theme
Using the sample code from documentation

**Package JSON:**

```
{
"name": "myproject",
"version": "0.0.0",
"scripts": {
"ng": "ng",
"start": "ng serve",
"build": "ng build",
"test": "ng test",
"lint": "ng lint",
"e2e": "ng e2e",
"css-build": "node-sass --omit-source-map-url sass/mystyles.scss css/mystyles.css"
},
"private": true,
"dependencies": {
"@angular/animations": "8.2.14",
"@angular/cdk": "8.2.3",
"@angular/common": "~8.2.14",
"@angular/compiler": "~8.2.14",
"@angular/core": "~8.2.14",
"@angular/forms": "~8.2.14",
"@angular/platform-browser": "~8.2.14",
"@angular/platform-browser-dynamic": "~8.2.14",
"@angular/router": "~8.2.14",
"@fortawesome/angular-fontawesome": "0.5.0",
"@fortawesome/fontawesome-svg-core": "^1.2.27",
"@fortawesome/free-solid-svg-icons": "^5.12.1",
"@nebular/theme": "^4.6.0",
"@swimlane/ngx-datatable": "^16.0.3",
"bulma": "^0.8.0",
"bulma-extensions": "^6.2.7",
"file-saver": "^2.0.2",
"ng2-completer": "^3.0.3",
"node-sass": "^4.13.1",
"rxjs": "^6.5.4",
"rxjs-compat": "^6.5.4",
"tslib": "^1.10.0",
"zone.js": "~0.9.1"
},
"devDependencies": {
"@angular-devkit/build-angular": "~0.803.25",
"@angular/cli": "~8.3.25",
"@angular/compiler-cli": "~8.2.14",
"@angular/language-service": "~8.2.14",
"@types/node": "~8.9.4",
"ts-node": "~7.0.0",
"tslint": "~5.15.0",
"typescript": "~3.5.3"
}
}
```