Simple lottery with debt collection at the end of grace period

Lottery.sol

pragma solidity ^0.4.17;

contract Lottery{
    uint public MAX_PLAYERS = 2;
    
    address[] public players;
    address public manager;
    uint public blockNumber;

    constructor() public{
        manager = msg.sender;
    }

    function enter() public payable{
        require(players.length < MAX_PLAYERS);
        require(msg.value == 1 ether);
        
        blockNumber = block.number;
        players.push(msg.sender);
    }

    function random() private view returns (uint){
        require(block.number > blockNumber);
        return uint(blockhash(blockNumber + 1));
    }

    function pickWinner() public restricted {
        address winner = players[random() % players.length];
        winner.transfer(address(this).balance);
        players = new address[](0);
    }
    
    function collectDebt() public restricted {
        require(msg.sender == manager);
        require((blockNumber + 3) < block.number);
        
        manager.transfer(address(this).balance);
        players = new address[](0);
    }

    function getPlayers() public view returns(address[]) {
        // Return list of players
        return players;
    }
    
    modifier restricted() {
        require(players.length == MAX_PLAYERS);
        _;
    }
}