Serial Event Driven - Fixed

const int maxChars = 1024;
int stringPosition = 0;
char myString[maxChars];
int counter = 0;

void serialEvent(){
  stringPosition = 0;
  while(Serial.available()){
    char data = (char)Serial.read();
    myString[stringPosition++] = data;
  }
  myString[stringPosition] = '\0';

  Serial.println(counter);
}

void setup(){
  Serial.begin(115200);
}

int divider = 0;

void loop(){
  if(divider++ % 100 == 0){
    counter++;
  }
}