subscriber unsubscribes just before a callback is made

Hi all

i have few subscribers listening for events, the problem is in some scenarios it unsubscribes, just before a call is made to it, thus throwing a null reference exception.

Psedo code

void InvokeCallback( )
{
   
   if(callback != NULL)
   {
      //At this point the subscriber unsubscribes and now the pointer is
    // null and it crashes with null reference exception,
   // can anyone tell me how to fix this issue.
      callback(...with paramerters..);
   }
}