Error: Call to a member function on null?

Question

PHP

Code:
public function donat ($player) {
$form = $api->createSimpleForm (function (Player $player int $data = null) {
if ($data === null) {
return;
}

Answer 1

For me, it’s written here in black and white what the problem is))
Also, as far as I know, the arguments should be separated by a comma ...
For me, it’s quite confusing: incomprehensible variables, calling a method, passing a function to its argument that takes its arguments of a certain class and number (but null by default). I would separate it all, but it doesn't matter.
The important thing is that there is no comma between Player $playerand int $data = null.

Answer 2

Where is the $api variable defined inside the function?