"Call to a member function createSimpleForm() on null"?

Question

PHP

public function donat ($player) {
        $api = $this->getServer()->getPluginManager()->getPlugin('FormAPI');
        $form = $api->createSimpleForm (function (Player $player, int $data = null) {
            if ($data === null) {
                return;
            }
            switch ($data) {
                case 0:
                    $this->fly ($player);
                break;
                case 1:

I DON'T UNDERSTAND WHAT'S WRONG

Answer 1

This means that $api is null, not an instance of the desired class, and it is not possible to call the createSimpleForm method.
In this method, $api appears out of thin air - it is neither passed from the outside nor created from within the method