Could not import backend for traitsui

Plz help!

I am using mavayi on ubuntu 18.04.1.

After I installed it via **pip install mayav**i and then **pip install PyQt5**.
I got the error like the title: Could not import backend for traitsui. Make sure you have a suitable UI toolkit like PyQt/PySide or wxPython installed.

Here is the environment:
python: 3.6.6
mayavi: 4.7.2
PyQt5: 5.15.1

Thanks a lot.