Arduino keeps crashing during serial print

[Idahowalker]

@dkanday

void onMqttMessage(int messageSize) {
  // we received a message, print out the topic and contents
  Serial.println("Received a message with topic '");
  Serial.print(mqttClient.messageTopic());
  Serial.print("', length ");
  Serial.print(messageSize);
  Serial.println(" bytes:");

You know that inside a ISR that printing serial.prints are a bad idea? Your ISR should use minimal code.