If you try and cast null to an int, you will get a NullPointerException.
If you want to null-check it, you can use an Integer or Object variable.
Integer r = (Integer) map.get("some_integer");
if (r==null) {
   // whatever you want to do in this case
} else {
   int result = r;
   // whatever you want to do with the result
}
solved Checking int cast from null