SelectButton onFocus throws error on Firefox

[SchneMa]

[x ] bug report => Search github for a similar issue or PR before submitting

Plunkr Case (Bug Reports)
instead of a Plunker:
https://www.primefaces.org/primeng/#/selectbutton

Current behavior
If you click the selectButton an Error occures as you can see in the console.
It seems like the button is still working after that. (I found now advice that something is not working like it should after the Error was thrown/logged).

ReferenceError: event is not defined
It seems like the Error occures in the onFocus event.
Error is shown even when that event is not used.

Tested in Firefox and Chrome
The Error occures in Firefox but not in Chrome.

Minimal reproduction of the problem with instructions
Go to the showcase Page (link above) and click on a selectButton while whatching to the console.

• PrimeNG version: Tested on 4.0.1

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
  FF

[andrush001]

In selectbutton.ts there is a missing parameter:

[andrush001]

It looks like there is a typo in your fix @cagataycivici

[sudheerj]

It was fixed in a new commit.