1.8.8 I don't know what happend?

I used a plugin's code.
but there are something i don't understand,and it's waring.

Code (Java):

public enum PackageType
{
     MINECRAFT_SERVER("net.minecraft.server." + getServerVersion()),
     //WARINGSTART
     CRAFTBUKKIT("org.bukkit.craftbukkit." + getServerVersion()),
     CRAFTBUKKIT_BLOCK((String)CRAFTBUKKIT, "block"),
     CRAFTBUKKIT_CHUNKIO((String)CRAFTBUKKIT, "chunkio"),
     CRAFTBUKKIT_COMMAND((String)CRAFTBUKKIT, "command"),
     CRAFTBUKKIT_CONVERSATIONS((String)CRAFTBUKKIT, "conversations"),
   CRAFTBUKKIT_ENCHANTMENS((String)CRAFTBUKKIT, "enchantments"),
     CRAFTBUKKIT_ENTITY((String)CRAFTBUKKIT, "entity"),
     CRAFTBUKKIT_EVENT((String)CRAFTBUKKIT, "event"),
     CRAFTBUKKIT_GENERATOR((String)CRAFTBUKKIT, "generator"),
     CRAFTBUKKIT_HELP((String)CRAFTBUKKIT, "help"),
     CRAFTBUKKIT_INVENTORY((String)CRAFTBUKKIT, "inventory"),
     CRAFTBUKKIT_MAP((String)CRAFTBUKKIT, "map"),
     CRAFTBUKKIT_METADATA((String)CRAFTBUKKIT, "metadata"),
     CRAFTBUKKIT_POTION((String)CRAFTBUKKIT, "potion"),
CRAFTBUKKIT_PROJECTILES((String)CRAFTBUKKIT, "projectiles"),
CRAFTBUKKIT_SCHEDULER((String)CRAFTBUKKIT, "scheduler"),
CRAFTBUKKIT_SCOREBOARD((String)CRAFTBUKKIT, "scoreboard"),
    CRAFTBUKKIT_UPDATER((String)CRAFTBUKKIT, "updater"),
     CRAFTBUKKIT_UTIL((String)CRAFTBUKKIT, "util");
   //WARINGEND
     private final String path;
 
     PackageType(String path) { this.path = path; }
 
     public String getPath() { return this.path; }
       public Class<?> getClass(String className) throws ClassNotFoundException { return Class.forName(this + "." + className); }
 
     public String toString() { return this.path; }
public static String getServerVersion() { return Bukkit.getServer().getClass().getPackage().getName().substring(23); }
}

Cannot cast from ReflectionUtils.PackageType to String
Exception:

Code (Text):

Task #1768 for cuilian v1.4 generated an exception
java.lang.Error: Unresolved compilation problem:

        at cc.toin.cuilian.lizi.ReflectionUtils$PackageType.getServerVersion(ReflectionUtils.java:296) ~[?:?]
        at cc.toin.cuilian.lizi.ParticleEffect$ParticlePacket.initialize(ParticleEffect.java:633) ~[?:?]
        at cc.toin.cuilian.lizi.ParticleEffect$ParticlePacket.<init>(ParticleEffect.java:596) ~[?:?]
        at cc.toin.cuilian.lizi.ParticleEffect.display(ParticleEffect.java:223) ~[?:?]
        at cc.toin.cuilian.lizi.Jianting.run(Jianting.java:76) ~[?:?]
        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftTask.run(CraftTask.java:71) ~[PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at org.bukkit.craftbukkit.v1_8_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:350) [PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:779) [PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:378) [PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:710) [PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:613) [PaperSpigot-1.8.8.jar:git-PaperSpigot-f6fba00-9935adc]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_181]

ReflectionUtils.java:296 -
public static String getServerVersion() { return Bukkit.getServer().getClass().getPackage().getName().substring(23); }