Null Pointer Exception with Arduino Communication

Processing Electronics (Arduino, etc.)
10

I have an Arduino Uno R3, and unfortunately, I don’t have any displays. What do you mean by:

I am looking at the function, and it’s only saying how many bytes are passing through. I think instead what you mean is to use “readStringUntil()” since the context makes a bit more sense for this. I have made the line-feed character a lowercase “c” just for testing purposes.
Heres is what I have for the Arduino now(still not working):

void loop() {
  //Serial.println("ok so the loop started");
  // put your main code here, to run repeatedly:
  while (Serial.available()>0){
    String MotorCode = Serial.readStringUntil("c");

    if (MotorCode.substring(0) == "C"){
      Claw.write(map(MotorCode.substring(1,-1).toInt(), 0, 180, 0, 180));
      digitalWrite(LED_BUILTIN,HIGH);
    }
    else {
      digitalWrite(LED_BUILTIN,LOW);
    }

  }

I have a feeling it has something to do with how I am decomposing the string, as I don’t know much about decomposing strings in Arduino.