[nbContextMenu] stopped working

### 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:**
I had to change template of NbMenuItem. To do this, I downloaded menu component sources from Nebulars github. Then I modified paths in imports to include new one.

Now clicking on [nbContextMenu] element throws an error (context menu is not opening):
```
NbPopoverComponent_Host.ngfactory.js? [sm]:1 ERROR TypeError: Cannot read property 'instance' of null
at NbPopoverComponent.set [as componentOutlet] (popover.component.js:35)
at checkAndUpdateQuery (core.js:12847)
at execQueriesAction (core.js:14207)
at checkAndUpdateView (core.js:13809)
at callViewAction (core.js:14153)
at execComponentViewsAction (core.js:14085)
at checkAndUpdateView (core.js:13808)
at callViewAction (core.js:14153)
at execEmbeddedViewsAction (core.js:14111)
at checkAndUpdateView (core.js:13803)
View_NbPopoverComponent_Host_0 @ NbPopoverComponent_Host.ngfactory.js? [sm]:1
proxyClass @ compiler.js:14653
DebugContext_.logError @ core.js:14996
ErrorHandler.handleError @ core.js:1509
(anonymous) @ core.js:5914
ZoneDelegate.invoke @ zone.js:392
Zone.run @ zone.js:142
NgZone.runOutsideAngular @ core.js:4697
ApplicationRef.tick @ core.js:5914
(anonymous) @ core.js:5740
ZoneDelegate.invoke @ zone.js:392
onInvoke @ core.js:4749
ZoneDelegate.invoke @ zone.js:391
Zone.run @ zone.js:142
NgZone.run @ core.js:4566
next @ core.js:5740
schedulerFn @ core.js:4331
SafeSubscriber.__tryOrUnsub @ Subscriber.js:239
SafeSubscriber.next @ Subscriber.js:186
Subscriber._next @ Subscriber.js:127
Subscriber.next @ Subscriber.js:91
Subject.next @ Subject.js:56
EventEmitter.emit @ core.js:4311
checkStable @ core.js:4714
onLeave @ core.js:4793
onInvokeTask @ core.js:4743
ZoneDelegate.invokeTask @ zone.js:424
Zone.runTask @ zone.js:192
ZoneTask.invokeTask @ zone.js:499
invokeTask @ zone.js:1540
globalZoneAwareCallback @ zone.js:1566

NbPopoverComponent_Host.ngfactory.js? [sm]:1 ERROR CONTEXT DebugContext_ {view: {…}, nodeIndex: 0, nodeDef: {…}, elDef: {…}, elView: {…}}
View_NbPopoverComponent_Host_0 @ NbPopoverComponent_Host.ngfactory.js? [sm]:1
proxyClass @ compiler.js:14653
DebugContext_.logError @ core.js:14996
ErrorHandler.handleError @ core.js:1514
(anonymous) @ core.js:5914
ZoneDelegate.invoke @ zone.js:392
Zone.run @ zone.js:142
NgZone.runOutsideAngular @ core.js:4697
ApplicationRef.tick @ core.js:5914
(anonymous) @ core.js:5740
ZoneDelegate.invoke @ zone.js:392
onInvoke @ core.js:4749
ZoneDelegate.invoke @ zone.js:391
Zone.run @ zone.js:142
NgZone.run @ core.js:4566
next @ core.js:5740
schedulerFn @ core.js:4331
SafeSubscriber.__tryOrUnsub @ Subscriber.js:239
SafeSubscriber.next @ Subscriber.js:186
Subscriber._next @ Subscriber.js:127
Subscriber.next @ Subscriber.js:91
Subject.next @ Subject.js:56
EventEmitter.emit @ core.js:4311
checkStable @ core.js:4714
onLeave @ core.js:4793
onInvokeTask @ core.js:4743
ZoneDelegate.invokeTask @ zone.js:424
Zone.runTask @ zone.js:192
ZoneTask.invokeTask @ zone.js:499
invokeTask @ zone.js:1540
globalZoneAwareCallback @ zone.js:1566

NbContextMenuComponent.html:1 ERROR Error: StaticInjectorError(AppModule)[NbMenuComponent -> NbMenuInternalService]:
StaticInjectorError(Platform: core)[NbMenuComponent -> NbMenuInternalService]:
NullInjectorError: No provider for NbMenuInternalService!
at _NullInjector.get (core.js:1002)
at resolveToken (core.js:1300)
at tryResolveToken (core.js:1242)
at StaticInjector.get (core.js:1110)
at resolveToken (core.js:1300)
at tryResolveToken (core.js:1242)
at StaticInjector.get (core.js:1110)
at resolveNgModuleDep (core.js:10854)
at NgModuleRef_.get (core.js:12087)
at resolveNgModuleDep (core.js:10854)
View_NbContextMenuComponent_0 @ NbContextMenuComponent.html:1
proxyClass @ compiler.js:14653
DebugContext_.logError @ core.js:14996
ErrorHandler.handleError @ core.js:1509
next @ core.js:5497
schedulerFn @ core.js:4331
SafeSubscriber.__tryOrUnsub @ Subscriber.js:239
SafeSubscriber.next @ Subscriber.js:186
Subscriber._next @ Subscriber.js:127
Subscriber.next @ Subscriber.js:91
Subject.next @ Subject.js:56
EventEmitter.emit @ core.js:4311
(anonymous) @ core.js:4771
ZoneDelegate.invoke @ zone.js:392
Zone.run @ zone.js:142
NgZone.runOutsideAngular @ core.js:4697
onHandleError @ core.js:4771
ZoneDelegate.handleError @ zone.js:396
Zone.runTask @ zone.js:195
ZoneTask.invokeTask @ zone.js:499
invokeTask @ zone.js:1540
globalZoneAwareCallback @ zone.js:1566
NbContextMenuComponent.html:1 ERROR CONTEXT DebugContext_ {view: {…}, nodeIndex: 1, nodeDef: {…}, elDef: {…}, elView: {…}}
```

Importing forRoot in app.module doesn't fix this:
https://github.com/akveo/ngx-admin/issues/1641
### Other information:

**npm, node, OS, Browser**
```
Node: v8.11.3
npm: 5.6.0
OS: macOS High Sierra
Browser: Chrome
```

**Angular, Nebular**
```
Angular: 5.2.6
Nebular:2.0.0-rc.7
```