`.type` and `.types` frame's attributes do not work properly in notebooks

When a notebook cell contains the following code
```python
DT = dt.Frame([1,2,3])
DT.type
```
it outputs
```python

```

The same code, when executed directly in python, works as expected
```python
>>> DT = dt.Frame([1,2,3])
>>> DT.type
Type.int32
```

`.types` behaves exactly the same.