Now doesn't crash without EssentialsPE :D

src/HealthBar/EventHandler.php

@@ -1,7 +1,6 @@
 <?php
 namespace HealthBar;
 
-use EssentialsPE\Events\PlayerNickChangeEvent;
 use pocketmine\event\entity\EntityDamageByEntityEvent;
 use pocketmine\event\entity\EntityDamageEvent;
 use pocketmine\event\entity\EntityRegainHealthEvent;
@@ -18,6 +17,14 @@ public function __construct(Loader $plugin){
         $this->plugin = $plugin;
     }
 
+    /**
+     * @param PlayerJoinEvent $event
+     */
+    public function onPlayerJoin(PlayerJoinEvent $event){
+        $player = $event->getPlayer();
+        $this->plugin->updateHealthBar($player);
+    }
+
     /**
      * @param EntityRegainHealthEvent $event
      */
@@ -61,20 +68,4 @@ public function onAttack(EntityDamageByEntityEvent $event){
             }
         }
     }
-
-    /**
-     * @param PlayerNickChangeEvent $event
-     */
-    public function onNickChange(PlayerNickChangeEvent $event){
-        $player = $event->getPlayer();
-        $nick = $event->getNewNick();
-        $nametag = $event->getNameTag();
-
-        $bar = $this->plugin->getHealthBar();
-        $bar = str_replace("maxhealth", $player->getMaxHealth(), $bar);
-        $bar = str_replace("health", $player->getHealth(), $bar);
-        $bar = str_replace("name", $nick, $bar);
-
-        $event->setNameTag($bar);
-    }
 } 

src/HealthBar/Loader.php

@@ -1,6 +1,7 @@
 <?php
 namespace HealthBar;
 
+use HealthBar\OtherEvents\EssentialsPEEvents;
 use pocketmine\Player;
 use pocketmine\plugin\Plugin;
 use pocketmine\plugin\PluginBase;
@@ -16,9 +17,11 @@ public function onEnable(){
         $this->getServer()->getCommandMap()->register("healthbar", new HealthBarCommand($this));
         $this->getServer()->getPluginManager()->registerEvents(new EventHandler($this), $this);
         $this->checkConfiguration();
+
         $ess = $this->getServer()->getPluginManager()->getPlugin("EssentialsPE");
         if($ess instanceof Plugin && $ess->isEnabled()){
             $this->essentialspe = new EssentialsPE();
+            $this->getServer()->getPluginManager()->registerEvents(new EssentialsPEEvents($this), $this);
         }
     }
 

src/HealthBar/OtherEvents/EssentialsPEEvents.php

@@ -0,0 +1,30 @@
+<?php
+namespace HealthBar\OtherEvents;
+
+use EssentialsPE\Events\PlayerNickChangeEvent;
+use HealthBar\Loader;
+use pocketmine\event\Listener;
+
+class EssentialsPEEvents implements Listener{
+    /** @var \HealthBar\Loader  */
+    public $plugin;
+
+    public function __construct(Loader $plugin){
+        $this->plugin = $plugin;
+    }
+
+    /**
+     * @param PlayerNickChangeEvent $event
+     */
+    public function onNickChange(PlayerNickChangeEvent $event){
+        $player = $event->getPlayer();
+        $nick = $event->getNewNick();
+
+        $bar = $this->plugin->getHealthBar();
+        $bar = str_replace("maxhealth", $player->getMaxHealth(), $bar);
+        $bar = str_replace("health", $player->getHealth(), $bar);
+        $bar = str_replace("name", $nick, $bar);
+
+        $event->setNameTag($bar);
+    }
+}