Cannot read property 'appendChild' of undefined

### 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:**

When a lazy loaded module is opened, a NbOverlayContainerAdapter gets constructed along with it which wont have its setContainer() called. This causes the nbDialog to fail in _createContainer()

**Expected behavior:**

setContainer() should be called, or createContainer() should append to document body if container undefined

**Steps to reproduce:**

I am using ngx-admin, I am calling an overlay container. Clicking that results in an error. I have looked up to nebular and the issue provided is to wrap app.component.html within . But Remember ngx-admin has its app.component template as just . The Pages module is wrapped within a layout template . The temporary solution provided shrinks my view content and it is all into a smaller window. Because i presume we have a layout inside a layout again. (ngx-sample-layout comes with its own tag with all the header and sidebar content.

**Related code:**
https://stackblitz.com/edit/nebular-overlay-container-adapter-bug?file=src%2Fapp%2Fapp.component.html

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### Other information:

**npm, node, OS, Browser**
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**Angular, Nebular**
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