mayavi run error

hello, i find a main error when running based on mayavi:

```
ImportError: Could not import backend for traitsui. Make sure you
have a suitable UI toolkit like PyQt/PySide or wxPython
```

I find this error many times. it is so crazy!!!

my installization process is

```
conda create -n vis python=3.7
pip install vtk==8.1.2
pip install PyQt5
pip install mayavi
```

i find
import mayavi # it is ok
from mayavi import mlab # it is ok

mlab.figure() # it is not ok -----ImportError: Could not import backend for traitsui. Make sure you have a suitable UI toolkit like
PyQt/PySide or wxPython

I try the PySide or wxPython, it's not ok also.

Can you help me find the solution?

Thanks~