My Code:

            Inventory anvilInv = Bukkit.createInventory(null, InventoryType.ANVIL);
            e.getWhoClicked().openInventory(anvilInv);

Error: (apperes if i click the inventory)

[11:28:42 FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.IndexOutOfBoundsException: Index: 26, Size: 0
        at java.util.concurrent.FutureTask.report(Unknown Source) ~[?:1.8.0_111]
        at java.util.concurrent.FutureTask.get(Unknown Source) ~[?:1.8.0_111]
        at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:45) [spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_111]
Caused by: java.lang.IndexOutOfBoundsException: Index: 26, Size: 0
        at java.util.ArrayList.rangeCheck(Unknown Source) ~[?:1.8.0_111]
        at java.util.ArrayList.get(Unknown Source) ~[?:1.8.0_111]
        at net.minecraft.server.v1_8_R3.Container.getSlot(Container.java:109) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:1463) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PacketPlayInWindowClick.a(SourceFile:31) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PacketPlayInWindowClick.a(SourceFile:9) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) ~[?:1.8.0_111]
        at java.util.concurrent.FutureTask.run(Unknown Source) ~[?:1.8.0_111]
        at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) ~[spigot-1.8.8.jar:git-Spigot-db6de12-18fbb24]
        ... 5 more