Need to also init the gravity links

src/space.c

@@ -399,6 +399,7 @@ void space_regrid(struct space *s, double cell_max, int verbose) {
       s->cells_top[k].density = NULL;
       s->cells_top[k].gradient = NULL;
       s->cells_top[k].force = NULL;
+      s->cells_top[k].grav = NULL;
       s->cells_top[k].dx_max = 0.0f;
       s->cells_top[k].sorted = 0;
       s->cells_top[k].count = 0;