#vulnerable code 
                              public static ClassName get(Class<?> clazz) {
    checkNotNull(clazz, "clazz == null");
    checkArgument(!clazz.isPrimitive(), "primitive types cannot be represented as a ClassName");
    checkArgument(!void.class.equals(clazz), "'void' type cannot be represented as a ClassName");
    checkArgument(!clazz.isArray(), "array types cannot be represented as a ClassName");

    String anonymousSuffix = "";
    while (clazz.isAnonymousClass()) {
      int lastDollar = clazz.getName().lastIndexOf('$');
      anonymousSuffix = clazz.getName().substring(lastDollar) + anonymousSuffix;
      clazz = clazz.getEnclosingClass();
    }
    String name = clazz.getSimpleName() + anonymousSuffix;

    if (clazz.getEnclosingClass() == null) {
      // Avoid unreliable Class.getPackage(). https://github.com/square/javapoet/issues/295
      int lastDot = clazz.getName().lastIndexOf('.');
      String packageName = (lastDot != -1) ? clazz.getName().substring(0, lastDot) : null;
      return new ClassName(packageName, null, name);
    }

    return ClassName.get(clazz.getEnclosingClass()).nestedClass(name);
  } 
                            #location 19 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              public static ClassName get(Class<?> clazz) {
    checkNotNull(clazz, "clazz == null");
    checkArgument(!clazz.isPrimitive(), "primitive types cannot be represented as a ClassName");
    checkArgument(!void.class.equals(clazz), "'void' type cannot be represented as a ClassName");
    checkArgument(!clazz.isArray(), "array types cannot be represented as a ClassName");

    String anonymousSuffix = "";
    while (clazz.isAnonymousClass()) {
      int lastDollar = clazz.getName().lastIndexOf('$');
      anonymousSuffix = clazz.getName().substring(lastDollar) + anonymousSuffix;
      clazz = clazz.getEnclosingClass();
    }
    String name = clazz.getSimpleName() + anonymousSuffix;

    if (clazz.getEnclosingClass() == null) {
      // Avoid unreliable Class.getPackage(). https://github.com/square/javapoet/issues/295
      int lastDot = clazz.getName().lastIndexOf('.');
      String packageName = (lastDot != -1) ? clazz.getName().substring(0, lastDot) : NO_PACKAGE;
      return new ClassName(packageName, null, name);
    }

    return ClassName.get(clazz.getEnclosingClass()).nestedClass(name);
  }