7 beta 1 breaking material mat-select

### Issue type

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

* [X] bug report
* [] feature request

### Issue description

**Current behavior:**
I have an angular 11 app and I would like to add the chat, so I'm adding Beta 1 of version 7. Adding the NbThemeModule to the app module using NbThemeModule.forRoot() gives me a problem with a few material selects I have in the app.
I have this:
```

Utenti

All


```
normally works fine, but as I add NbThemModule, when I click the open the list I have this error in the console

> ERROR TypeError: this.container is undefined

and

> ERROR TypeError: this._overlayRef is undefined
I'm using v 11.0.3 of @angular/material

**Expected behavior:**

To be able to use the material select
**Steps to reproduce:**

add the code in an angular app with material and click in the select to open it

### Other information:

**npm, node, OS, Browser**
```
Node 12.13.0, npm 6.12.0:
OS: Windows 10.
Browser: Chrome Firefox-->
```

**Angular, Nebular**
```
angular 11.0.3
nebular 7 beta 1
angular/material 11.0.3

```