Null Pointer Exception with Arduino Communication

Processing Electronics (Arduino, etc.)
6

Ok, I am now able to get it responding to the software, but it only works well with single characters. Is it just how I am formatting the strings, or can the Arduino only receive one character at a time through the Serial? It only seems to work if I use single characters.
Arduino:

while (Serial.available()>0){
    String MotorCode = Serial.readString();

    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);
    }

  }

Processing:

void draw() {
  getUserInput();
  background(selectcontrol,700,400);
  println("You are now in Control >:)");
  
  //send a list that consists of the joint's name and, position(and direction) to the serial for the arduino
  if(clawarmselect == true){ //control claw arm with right stick
    println("Selecting the claw arm to control, use the right stick to control");
    portmessage = "A"+str((int)selectcontrol);
    println(portmessage);
    port.write(portmessage);
  }
  
  if(elbowselect == true){ //control elbow with button and control right stick
    println("Selecting the elbow to control, use the right stick to control");
    portmessage = "B"+str((int)selectcontrol);
    println(portmessage);
    port.write(portmessage);
  }
  
  if(clawgrab == true){ //close claw
    println("Claw Closing");
    port.write("C90");
  }
  
  if(clawrelease == true){ //open claw
    println("Claw Opening");
    port.write("C180");
  }