nb-checkbox uncheck all not working with reactive forms

### Issue type

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

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**

nb-checkbox , uncheck all dosent work when checking 'None of above', hence i am able to remove from submitted form value but i cant be able to uncheck them in view.
**Expected behavior:**

**Steps to reproduce:**

**Related code:**

```

onltsChange(value,e){
const lifeThreateningSymptoms = this.ltsGetter;
if(e){
if(value === 'none'){
lifeThreateningSymptoms.controls.forEach((items:FormControl) => {
while(lifeThreateningSymptoms.length !== 0){
lifeThreateningSymptoms.removeAt(0);
}
});
}
lifeThreateningSymptoms.push(new FormControl(value));
}else{
let i:number = 0;
lifeThreateningSymptoms.controls.forEach((item:FormControl) => {
if(item.value == value){
lifeThreateningSymptoms.removeAt(i);
return;
}
i++;
})
}
}
```

### Other information:

**npm, node, OS, Browser**
```
Node version : v12.16.1
NPM version : 6.13.4
OS : macOS Mojave
Browser : Chrome 80.0.3987.149
```

**Angular, Nebular**
```
Angular : 9.0.6
Nebular : 5.0.0
```