How can an already-defined function pointer suddently become null and throw a segfault?

This is a piece of code, which did work, but suddenly started throwing segfaults after I had changed a library function call in a different piece of code. Here is what gdb says:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x75bae3d0 (LWP 756)]
0x00000000 in ?? ()

Here are the relevant chunks of code, which cause the issue:

void relay::IgnitionISR (int handler, unsigned int pin, unsigned int level, uint32_t tick)
{
  cout << "¤¤ " << relay::ignition_on_off_event_handler << " ¤¤" << endl;

  if (level == 1) {
    keypad::SetLock (keypad::ON_OFF);
    relay::ignition_on_off_event_handler (true, tick);
  }
  else {
    keypad::SetLock (keypad::LOCKED);
    relay::ignition_on_off_event_handler (false, tick);
  }
}

When this method is called by an external trigger (see below), I always get this output:

¤¤ 0 ¤¤

The event handler is declared in the header file like this:

typedef void (*ignition_event_handler) (bool, uint32_t);

private:
  static ignition_event_handler ignition_on_off_event_handler;

  static void IgnitionISR (int handler, unsigned int pin, unsigned int level, uint32_t tick);

The function pointer is set in this method:

int relay::Initialize (int pi_handler, ignition_event_handler ign_handler)
{
  int  result = 0;
  bool is_ok  = true;

  relay::pigpiod_handler               = pi_handler;
  relay::ignition_on_off_event_handler = ign_handler;

  result = callback (relay::pigpiod_handler, IGNITION_PIN, EITHER_EDGE, (CBFunc_t) relay::IgnitionISR);
  // This method from pigpiod_if2.h essentially makes the ISR being called once the state of the pin given changes.

  if (result == pigif_bad_malloc || result == pigif_duplicate_callback || result == pigif_bad_callback) {
    cout << "ERROR on pin " << IGNITION_PIN << "! Code: " << result << ".";

    is_ok = false;
  }

  cout << "¤¤ " << relay::ignition_on_off_event_handler << " ¤¤" << endl;
  return result;
}

In the past (when I used to address pigpio directly), I was using this method from pigpio.h instead of callback (), and the event handler worked well:

gpioSetAlertFunc (IGNITION_PIN, IgnitionISR);

This is how the initializing method is called from the main () procedure in main.c:

ret = relay::Initialize (pigpiod_handler, OnIgnitionSwitched);

And this is how the handler function itself (whose pointer is used) is defined in main.c:

void OnIgnitionSwitched (bool is_on, uint32_t tick) {…}

From the initializing method, I always get this output:

¤¤ 1 ¤¤

And here comes my problem: How can the function pointer value suddenly become 0 when triggering the ISR, as the corresponding variable is private, and no other piece of code from within the class writes to it? Thank you for any useful hints!