workspace thumbnail will concide after open it many times

[vhtq18w]

Distro: Ubuntu Focal (development branch)
Architecture: aarch64
Scope of influence: 20200314 build~20200320 build

Reproduce
It can only be reproduced on the arm platform. First, install ubuntu focal 20.03 via official daily image or rootfs, make sure gnome desktop is installed. Then install the latest ukui desktop environment, click the ukui-window-switch button at least twice after startup, you can see workspace have a pure black background and it's thumbnail will coincide.

These conditions works fine:

• installation with official ubuntukylin amd64 cdimage
• all amd64 release