(struct.h)

typedef struct      s_cell
{
    char            cell;
    unsigned int    index;
}                   t_cell;

typedef struct      s_grid
{
    size_t          width;
    size_t          height;
    char            *str;
    char            vide;
    char            obs;
    char            plein;
    struct s_cell   **grid;
}                   t_grid;

...

(grid_init.c)
#include "struct.h"

...

grid[j][i] = malloc(height * sizeof(t_cell));

if (grid[j][i] == NULL)