Please see the minimized test below to reproduce the issue.

Minimized test:
===============
$ cat Test.java
import java.lang.invoke.*;

public class Test {
    public static void main(String[] args) throws Exception {
        MethodType.fromMethodDescriptorString(
                (String) null, ClassLoader.getSystemClassLoader());
    }
}

Minimized test output:
========================
$ javac Test.java

$java -showversion -verify Test
java version "1.7.0-ea"
Java(TM) SE Runtime Environment (build 1.7.0-ea-b138)
Java HotSpot(TM) Server VM (build 21.0-b08, mixed mode)

Exception in thread "main" java.lang.NullPointerException
        at sun.invoke.util.BytecodeDescriptor.parseMethod(BytecodeDescriptor.java:41)
        at java.lang.invoke.MethodType.fromMethodDescriptorString(MethodType.java:717)
        at Test.main(Test.java:5)
NPE is also thrown for descriptor "(.)"