Internationalization failing

### Issue type

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

* [x ] bug report
* [ ] feature request

### Issue description
Using Angular 10 and nebular 6 when I run the basic command "ng xi18n" I get an error because I have overridden the authentication getter for errors coming from my backend rest services.

errors: {
// This overrides the getter of errors functions
// res: It is the HttpResponse coming from backend
getter: (module, res, options) => {
return res.error ? res.error.msg.Message_ : options[module].defaultErrors;
},

**Current behavior:**

I receive the following error if I just run the international command:

ERROR in Error during template compile of 'AppModule'
Function expressions are not supported in decorators in 'ThemeModule'
'ThemeModule' contains the error at app/@theme/theme.module.ts(168,25)
Consider changing the function expression into an exported function.

**Expected behavior:**

The proper i18n files would be created in my application.

**Steps to reproduce:**

Just run ng xi18n and the error will appear.

**Related code:**

```
insert short code snippets here
```

### Other information:

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