Comment 8 for bug 1805476

Those kinds of masks typically appear when an object is exported to pdf, and then imported again. They do not appear on their own. Have you added them yourself?

It may be that you accidentally saved as or opened the file as pdf.

It's good that it's not crashing anymore, let's hope it will stay that way... If it reappears, you can create a new report for it anytime.

I'm think that the mask/pdf export issue has been reported multiple times, even. One of them is here: https://bugs.launchpad.net/inkscape/+bug/427540