Fixed kotlin class names

eco-core/core-plugin/src/main/kotlin/com/willfp/eco/spigot/EcoSpigotPlugin.kt

@@ -56,7 +56,7 @@ abstract class EcoSpigotPlugin: EcoPlugin(
         CustomItemsManager.registerAllItems()
     }
 
-    override fun loadIntegrationLoaders(): List<IntegrationLoader?> {
+    override fun loadIntegrationLoaders(): List<IntegrationLoader> {
         return listOf( // AntiGrief
                 IntegrationLoader("WorldGuard") { AntigriefManager.register(AntigriefWorldGuard()) },
                 IntegrationLoader("GriefPrevention") { AntigriefManager.register(AntigriefGriefPrevention()) },