nb-tree-grid-row-toggle ERROR TypeError: Cannot read property 'elementRef' of undefined

### Issue type

* [x] bug report
* [ ] feature request

### Issue description
Using TreeGrid, sometimes when I click the chevron icon (right arrow) this error appears:

`ERROR TypeError: Cannot read property 'elementRef' of undefined
at oa.getDataIndex (main-es2015.1d739b58e35289c1defe.js:1)
at oa.toggleRow (main-es2015.1d739b58e35289c1defe.js:1)
at oa.toggleCellRow (main-es2015.1d739b58e35289c1defe.js:1)
at Oa.toggleRow (main-es2015.1d739b58e35289c1defe.js:1)
at toggleRow (main-es2015.1d739b58e35289c1defe.js:1)
at Object.handleEvent (1-es2015.1eb53515f8fbaee146a1.js:1)
at Object.handleEvent (main-es2015.1d739b58e35289c1defe.js:1)
at Object.handleEvent (main-es2015.1d739b58e35289c1defe.js:1)
at Ei (main-es2015.1d739b58e35289c1defe.js:1)
at main-es2015.1d739b58e35289c1defe.js:1`

It does not occur when I click the item inside the grid. Just the arrow icon which was generated by the nb-tree-grid-row-toggle

NOTE: Data is being served by the server, not static.

**Current behavior:**
Error shows sometimes. Mostly on first click.

**Expected behavior:**

**Steps to reproduce:**

**Related code:**

```

data: TreeNode[];

constructor(private coaService: CoaService) {
this.getCoas();
}

getCoas() {
this.coaService.getCoas().subscribe(coas => {
this.data = coas;
})
}

{{row.data.name}}

{{row.data[column]}}

```

### Other information:

**npm, node, OS, Browser**
```
OS: Windows 10
node version v10.15.3
Browser: Chrome
```

**Angular, Nebular**
```
"@angular/animations": "^8.2.3",
"@angular/cdk": "^8.2.3",
"@angular/common": "~8.2.10",
"@angular/compiler": "~8.2.10",
"@angular/core": "~8.2.10",
"@angular/forms": "~8.2.10",
"@angular/platform-browser": "~8.2.10",
"@angular/platform-browser-dynamic": "~8.2.10",
"@angular/router": "~8.2.10",
"@fortawesome/fontawesome-free": "^5.8.2",
"@nebular/theme": "^4.6.0",
"bootstrap": "^4.3.1",
"core-js": "^2.5.4",
"rxjs": "~6.5.3",
"tslib": "^1.9.0",
"zone.js": "~0.9.1"
```