System Information
Operating system: Linux-5.10.14-arch1-1-x86_64-with-glibc2.33 64 Bits
Graphics card: GeForce RTX 2070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.39

Blender Version
Broken: version: 2.93.0 Alpha, branch: master, commit date: 2021-02-12 15:54, hash: rBc04bcc87fe72
Worked: Probably never? Feature was added in 1352d81b174726639bbfb6f7aa32dbadf188a8dd

Short description of error
Selecting multiple Grease Pencil keyframes with Multiframe enabled in draw mode crashes when the timeline marker is between the selected frames.

Exact steps for others to reproduce the error

Quick reproduction steps:

1. Open provided blend file.
2. Shift-select the keyframe on frame 20 in the dope sheet
3. Blender crashes.

From startup:

1. Open new file with 2D Animation template.
2. Move to frame 20 and click the '+' button on the dope sheet, adding a new layer (and keyframe).
3. Turn on Multiframe while still in draw mode.
4. Select keyframe on frame 1.
5. Shift-Select keyframe on frame 20.
6. Move to a frame between 1 and 20 (in the yellow bits). Note it doesn't crash when the timeline marker is outside of that frame range
7. Blender crashes.