Fixed GUI and rendering bugs

eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/menu/MenuRenderedInventory.kt

@@ -17,8 +17,8 @@ class MenuRenderedInventory(
     fun render() {
         generateCaptive()
 
+        var i = 0
         for (row in menu.slots) {
-            var i = 0
             for (slot in row) {
                 if (i == menu.rows * 9) {
                     break

eco-core/core-backend/src/main/kotlin/com/willfp/eco/internal/gui/slot/EcoSlot.kt

@@ -6,6 +6,7 @@ import com.willfp.eco.core.gui.slot.functional.SlotHandler
 import com.willfp.eco.core.gui.slot.functional.SlotProvider
 import com.willfp.eco.core.gui.slot.functional.SlotUpdater
 import com.willfp.eco.internal.gui.menu.getMenu
+import org.bukkit.Material
 import org.bukkit.entity.Player
 import org.bukkit.event.inventory.ClickType
 import org.bukkit.event.inventory.InventoryClickEvent
@@ -39,7 +40,7 @@ open class EcoSlot(
     override fun getItemStack(player: Player): ItemStack {
         val menu = player.openInventory.topInventory.getMenu()!!
         val prev = provider.provide(player, menu)
-        return updater.update(player, menu, prev)
+        return updater.update(player, menu, prev) ?: ItemStack(Material.AIR)
     }
 
     fun getItemStack(
@@ -47,7 +48,8 @@ open class EcoSlot(
         menu: Menu
     ): ItemStack {
         val prev = provider.provide(player, menu)
-        return updater.update(player, menu, prev)
+        val updated = updater.update(player, menu, prev)
+        return updated ?: ItemStack(Material.AIR)
     }
 
     override fun isCaptive(): Boolean {

eco-core/core-plugin/src/main/kotlin/com/willfp/eco/internal/spigot/gui/GUIListener.kt

@@ -45,6 +45,7 @@ class GUIListener(private val plugin: EcoPlugin) : Listener {
         }
 
         val menu = inv.getMenu() ?: return
+        val rendered = inv.asRenderedInventory() ?: return
 
         val (row, column) = MenuUtils.convertSlotToRowColumn(inv.firstEmpty())
 
@@ -53,6 +54,8 @@ class GUIListener(private val plugin: EcoPlugin) : Listener {
         if (!slot.isCaptive) {
             event.isCancelled = true
         }
+
+        plugin.scheduler.run { rendered.render() }
     }
 
     @EventHandler(priority = EventPriority.HIGH)