subsetGiotto error

when subset Giotto with cell ids, it thraw an error

```
gobj_sub <- subsetGiotto(gobject = gobj, 
    cell_ids = as.character(oligo_ids))
```

this type of subset also generate this error

```
gobj[, (gobj$cell_ID %in% oligo_ids)]
```

错误于h(simpleError(msg, call)): 
  在为函数“as.list”选择方法时计算参数“x”时出错：NULL值不能当作符号地址用

Giotto version: 4.2.2
GiottoClass version: 0.4.9