Using library functions in timer ISR ESP32

I'm trying to use a library function in my ISR function that uses the internal timer to trigger every 1 ms. The chip that I'm using is an ESP32-C3-MINI. Here is my current code:

main.cpp

#include <Arduino.h>
hw_timer_t *_timer = NULL;
myClass a();

void IRAM_ATTR isr()
{
    a.incrementCount();
}


void setup()
{
    coin_timer = timerBegin(0, 80, true);
    timerAttachInterrupt(_timer, &isr, true);
    timerAlarmWrite(_timer, 1000, true);
    timerAlarmEnable(_timer);
}

myClass.h

#include <Arduino.h>

class myClass
{
private:
    unsigned long count;

public:
    myClass();
    ~myClass();

    void incrementCount();  
}

myClass.cpp

#include <Arduino.h>
#include "myClass.h"

myClass::myClass(){

}

void myClass::incrementCount(){
    count++;
}

This is just an example of the code that I wrote. In reality, the incrementCount function is a little more complicated and involves multiple variables and arrays within the library. This current code compiles and works without an issue. However, I would like to know if this method is bad practice or can cause problems in certain situations. Secondly, I notice if I declare count variable as volatile, the MCU just crashes and enters a reboot cycle.