System Information
Operating system: Linux-5.8.4-200.fc32.x86_64-x86_64-with-fedora-32-Thirty_Two 64 Bits
Graphics card: GeForce GTX 970M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.66

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-10-17 21:36, hash: rBf425f40c4e17
Worked: 2.90.1
Caused by rB6faa765af895: Sculpt: Split Box Mask into its own operator

Short description of error
Calling Box Mask from the Mask menu or using B will not result in a sculpt mask

Exact steps for others to reproduce the error
from the default startup file:

• enter sculpt mode
• try to draw a mask on the Cube (calling Box Mask from the Mask menu or using B)
• no resulting sculpt mask

Since rB6faa765af895: Sculpt: Split Box Mask into its own operator, we have to use paint.mask_box_gesture instead now.