ERROR TypeError: "this.container is undefined" for NbDialog

Issue type

I'm submitting a
* [ ] bug report
* [ ] feature request
* Unknown cause, Not qualified enough to call this a bug, & certainly not a feature request. Just that I am not able to get NbDialog working because of this not much descriptive error & my incapability to understand the complicated stack trace.

Issue description
NbDialog service is not able to open a dialog box by giving an error " ERROR TypeError: "this.container is undefined" for NbDialog"

Steps to reproduce:
https://angular-acregw.stackblitz.io

Kindly help by giving it a look.