[STL::Map] Error occured

#include <iostream>
#include <map>

int main()
{
 std::map<int, int> mapTest ;
 
 for (int i = 0 ; i <10 ; i++)
 {
  mapTest[i] = 10*i ;
 }
 std::map<int, int>::iterator mapIt ;

 mapIt = mapTest.find(10) ;

 /*
 if (mapIt != NULL)
  std::cout << "mapIt = " << mapIt->second << std::endl ;
 */ // Error occur: the iterator might not find out its error.

if (mapIt != mapTest.end())
  std::cout << "mapIt = " << mapIt->second << std::endl ;

 int i ;
 std::cin >> i ;

 return 0 ;
}

My friend saw my source code, and adviced to me that the code will be broken from map's iterator.

So, I simply tested this source. I could find out where the problem occured.

However, I can't explain why the error occurs. I need to study about STL, especially map.

- ps.
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