Defencive style nightmare

gistfile1.java

/**
 * Extracts digits from string
 * returns digits string or null if nothing found
 */
@Nullable String getDigits(String str) {
  return getDigitsInternal(str)  
}

/**
 * Multiplies string by 2
 */
String dobleString(String str) {
  if(str != null) {
    return str + str;
  }
  return null;
}

String toUpperCase(String str) {
  if(str != null) {
    return str.toUpper();
  }
  return null;
}

public static void main() {
  String str = "abc"; // <-- contains no digits
  String digits = getDigits(str);
  sendWithTcp(toUpperCase(doubleString(digits))); // <-- NPE in send() method
}

Happy Debug mode is ON