i tried downloading client warpper but when i join a server or try generate a level i get this

sessionid: -1859909972391589738
Exception in thread "main" java.lang.VerifyError: class KPlayer overrides final
method tick.()V
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClassCond(Unknown Source)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$000(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at MainApplet.init(MainApplet.java:96)
        at Main.main(Main.java:61)
Failed to initialize device USB Wireless HID Receiver because of: java.io.IOExce
ption: Failed to acquire device (8007001e)
java.lang.NullPointerException
        at java.applet.Applet.getCodeBase(Unknown Source)
        at com.mojang.minecraft.l.a(Unknown Source)
        at com.mojang.minecraft.l.a(Unknown Source)
        at com.mojang.minecraft.l.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)