The method ConstantPool.getBootstrapMethodAt(int) can return null:

public short[] getBootstrapMethodAt(int bsmIndex) {
    U4Array offs = getOffsets();
    U2Array bsms = getBootstrapMethods();
    if (offs == null || bsms == null) return null; // safety first
    ...
}

However, ClassWriter.writeClassAttributes() calls this method without checking for null:

for (int index = 0; index < bsmCount; index++) {
    short[] value = cpool.getBootstrapMethodAt(index);
    for (int i = 0; i < value.length; i++) {
        ...
To prevent possible NPE, getBootstrapMethodAt() should return an empty array instead of null.