Invalid Java Runtime Configuration ERROR code: 1

[18:29:55] [main/INFO]: Loading tweak class name net.minecraftforge.fml.common.launcher.FMLTweaker
[18:29:55] [main/INFO]: Using primary tweak class name net.minecraftforge.fml.common.launcher.FMLTweaker
[18:29:55] [main/INFO]: Loading tweak class name net.digitalingot.feather.launch.tweaker.FeatherTweaker
[18:29:55] [main/ERROR]: Unable to launch
java.lang.ClassNotFoundException: net.digitalingot.feather.launch.tweaker.FeatherTweaker
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[?:1.8.0_51]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331) ~[?:1.8.0_51]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
    at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:106) ~[launchwrapper-1.12.jar:?]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
    at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51]
    at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51]
    at net.minecraft.launchwrapper.Launch.launch(Launch.java:98) [launchwrapper-1.12.jar:?]
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]

Pls help me.