Ошибка исполнения "Expression: stream != nullptr" - C (СИ)

Формулировка задачи:

Expression: stream != nullptr For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

                    Листинг программы
                
 
                            
                        
# include <stdio.h>
# include <conio.h>
# include <stdlib.h>
# include <locale.h>
FILE *fp;
 
struct list1
{
    char elem1;
    list1 *next, *pred;
} *ph;
list1 *p, *H;
char sym;
 
struct list2
{
    char elem2;
    list2 *next;
};
 
list2 *search(list1 *ph, char sym2)
{
    list2 *t, *R;
    list1 *p = ph;
    R = new list2;
    R->next = NULL;
    t = R;
    do {
        if (p->elem1 == sym2)
        {
            t->next = new list2;
            t->next->next = new list2;
            t = t->next;
            t->elem2 = p->pred->elem1;
            t = t->next;
            t->next = NULL;
            t->elem2 = p->next->elem1;
        }
        p = p->next;
    } while (p != ph);
    return R;
};
void main()
{
    char c = 'c';
    list2 *L, *lp = NULL;
    setlocale(LC_CTYPE, "Rus");
 
    { ph = new list1;
    ph->pred = NULL;
    ph->next = NULL;
    p = ph;
    printf("Введите последовательность символов\n");
    fscanf_s(fp, "%c", &sym);
    while (sym != '.')
    {
        p->next = new list1;
        p = p->next;
        p->elem1 = sym;
        fscanf_s(fp, "%c", &sym);
    }
    p->next = NULL;
    p->pred = NULL;
    }
    printf("Введите символ \n");
    scanf_s("%c", c);
    L = search(H, c);
    for (L = lp; lp != NULL; lp = lp->next);
    fprintf(fp, "%c", lp->elem2);
    _getch();
}

Решение задачи: «Ошибка исполнения "Expression: stream != nullptr"»

textual
                            Листинг программы 
                        
L = search(H, c);