Why NoClassDefFoundError caused by static field initialization failure?

Here is a interesting java question.

the following simple java program contains static field initialized by a method statically. Actually, I force the method which calculate the intiailize value to raise a NullPointException, When I access such a static field, a NoClassDefFoundError will raised. it seems the VM treat the Class is not complete.

But when I access the Class, it still available;

Does anyone knows why?

class TestClass {
    public static TestClass instance = init();

    public static TestClass init() {
       String a = null;
       a.charAt(0); //force a null point exception;
       return new TestClass();
    }
}

class MainClass {
    static public void main(String[] args) {
       accessStatic(); // a ExceptionInInitializerError raised cause by NullPointer
       accessStatic(); //now a NoClassDefFoundError occurs;

       // But the class of TestClass is still available; why?
       System.out.println("TestClass.class=" + TestClass.class);
    }

    static void accessStatic() {
        TestClass a;

        try {
            a = TestClass.instance; 
        } catch(Throwable e) {
            e.printStackTrace();
        }
    }   
}