Crashed Plugin rankUp

Code:

PocketMine-MP Crash Dump Fri Aug 18 09:07:57 CEST 2017

Error: Declaration of rankup\command\RankUpCommand::getPlugin() must be compatible with pocketmine\command\PluginIdentifiableCommand::getPlugin(): pocketmine\plugin\Plugin
File: RankUp-master/src/rankup/command/RankUpCommand
Line: 10
Type: E_COMPILE_ERROR

THIS CRASH WAS CAUSED BY A PLUGIN
BAD PLUGIN: RankUp v0.4.0

Code:
[1] <?php
[2] namespace rankup\command;
[3] 
[4] use pocketmine\command\Command;
[5] use pocketmine\command\CommandSender;
[6] use pocketmine\command\PluginIdentifiableCommand;
[7] use pocketmine\Player;
[8] use rankup\RankUp;
[9] 
[10] class RankUpCommand extends Command implements PluginIdentifiableCommand{
[11]     private $main;
[12]     public function __construct(RankUp $main){
[13]         parent::__construct("rankup", "Get all the ranks.", "/rankup", ["ru"]);
[14]         $this->main = $main;
[15]     }
[16]     public function execute(CommandSender $sender, string $label, array $args): bool{
[17]         if ($sender instanceof Player && count($args) == 0 || !$sender->hasPermission("rankup.admin")) {
[18]             if ($sender->hasPermission("rankup.rankup")) {
[19]                 $nextRank = $this->getPlugin()->getRankStore()->getNextRank($sender);
[20]                 //$sender->sendMessage($nextRank->getName());

Backtrace:

PocketMine-MP version: 1.6.2dev #311 [Protocol 113; API 3.0.0-ALPHA7]
Git commit: 7d777510c5cee564bdd9f6d1e55d2d6c04e7de16
uname -a: Linux fr24.server.pro 3.14.32-xxxx-grs-ipv6-64 #6 SMP Wed Jan 20 17:52:44 CET 2016 x86_64
PHP Version: 7.0.9
Zend version: 3.0.0
OS : Linux, linux

Loaded plugins:
DevTools 1.12.1 by PocketMine Team for API(s) 3.0.0-ALPHA7
AdvancedKits 5.2.0 by luca28pet for API(s) 3.0.0-ALPHA7
RankUp 0.4.0 by Falkirks for API(s) 3.0.0-ALPHA7

Why still keep crashed I changed code..

 

There are still some typehints you missed.

 

How I find missed code again?

 

It tells you in the crash dump.