HELP! Kenapa hasil parsing null atau [ ] ?

<a href='https://ibb.co/ySgmq9D'>Postman result</a><?php
	//DB connection using variable-> $con
	require_once 'config.inc.php';


	$response = array();

	if(isset($_REQUEST['apicall'])){

		switch($_REQUEST['apicall']){


			case 'login':

				if(isTheseParametersAvailable(array('userID', 'password'))){
					//$id 	  =  $_POST['idUser']
					$username = $_REQUEST['userID'];
					$password = $_REQUEST['password'];

					$stmt = $con->prepare("SELECT userID, password FROM tbl_userprospek WHERE userID = ? AND password = ?");
					$stmt->bind_param("ss", $username, $password);

					$stmt->execute();

					$stmt->store_result();

					if($stmt->num_rows > 0){

						$stmt->bind_result($username, $password);
						$stmt->fetch();

						$user = array(

							'userID'=>$username,
							'password'=>$password,

						);

						$response['error'] = false;
						$response['message'] = 'success';
						$response['user'] = $user;
					}else{
						$response['error'] = false;
						$response['message'] = 'Invalid username or password';
					}
				}
			break;

			case 'register':

				if(isTheseParametersAvailable(array('userID','email','password'))){
					$username = isset($_POST['userID']);
					$email = isset($_POST['email']);
					$password = isset($_POST['password']);
					//$gender = $_POST['gender'];

					$stmt = $con->prepare("SELECT idUser FROM tbl_userprospek WHERE userID = ? OR email = ? ");
					$stmt->bind_param("ss", $username, $email);
					$stmt->execute();
					$stmt->store_result();
					if($stmt->num_rows > 0){

						$response['error'] = true;
						$response['message'] = 'User already registered';
						$stmt->close();
					}else{
						//if user is new creating an insert query
						$stmt = $con->prepare("INSERT INTO tbl_userprospek (userID, email, password) VALUES (?, ?, ?)");
						$stmt->bind_param("sss", $username,  $email, $password);

						//if the user is successfully added to the database
						if($stmt->execute()){

							//fetching the user back
							$stmt = $con->prepare("SELECT idUser, userID, password, email FROM tbl_userprospek WHERE userID = ?");
							$stmt->bind_param("s",$username);
							$stmt->execute();
							$stmt->bind_result($username, $email, $password);
							$stmt->fetch();


							$user = array(
								'idUser'=>$id,
								'userID'=>$username,
								'email'=>$email,
								'password'=>$password
							);

							$stmt->close();

							//adding the user data in response
							$response['error'] = false;
							$response['message'] = 'User registered successfully';
							$response['user'] = $user;
						}
					}

				}else{
					$response['error'] = true;
					$response['message'] = 'required parameters are not available';
				}

			break;


			default: //default result
				$response['error'] = true;
				$response['message'] = 'Invalid Operation Called';
		}//end case
	//if params
	}else{
		//if it is not api call
		//pushing appropriate values to response array
		$response['error'] = true;
		$response['message'] = 'Invalid API Call';
	}

	echo json_encode($response);


	//function validating all the paramters are available
	//we will pass the required parameters to this function
	function isTheseParametersAvailable($params){
		//traversing through all the parameters
		foreach($params as $param){
			///if the paramter is not available
			if(!isset($_REQUEST[$param])){
				return false;
			}
		}
		//return true if every param is available
		return true;
	}