UE4 Editor crashed whenever saving UI widget blueprint

I’m using UE 4.20.3 on Mac OS 10.13.6 with graphic card AMD FirePro D700 6144 MB

The project works fine until I add or change a few buttons in widget blueprint, and then out of blue, pressing “Save” caused crashes. My workaround is to rebuild a brand new widget BP from scratch ( with exactly the same widgets and blueprints ), but this becomes not practical when the BP grows complicated… Any idea? Thanks.

It turns out the widget BP in questions are in different levels, and if I switch to the level they belongs to, the problem is gone. Cheers!