Is this line mistyped or so? its throwing some error

Line 48 in 7c54c2b

public function getResponseData(): ?array

Why do you think it's a typo? What error is it throwing? Please elaborate.

I noticed an error saying unexpected '?' or so

Is this part valid / is it any shorthand operator? - : ?array

Shouldn't it be { after function()

The ? is part of the return type hint. It says that it returns an array, the question mark says it can also return null.

I didn't notice this is only added in PHP 7.1, while the library still supports PHP 5.2. Are you using a PHP version older than 7.1?
I will change this when I have the opportunity.

Yes. Noticed it on older php :)

Fixed for PHP 5 now in version 1.2.2.