Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 2070 with Max-Q Design/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 466.63
Broken: version: 2.93.0, branch: master, commit date: 2021-06-02 11:21, hash: rB84da05a8b806
Short description of error
Segmentation fault in anim_channels_edit.c > join_groups_action_temp. (access to a nullptr'd variable).
Exact steps for others to reproduce the error
Big mystery. I had this happen in a big file, and removed everything until I only had the plane that is in the file that is linked to this bug report.
I then added an armature with one bone, and keyed it twice. Trying to use the "move up/down/top/bottom" commands on the corresponding channel (named "Reorder me!") will crash blender.
I have submitted a fix for this bug, but I am afraid the problem might be deeper than what is being fixed in the patch.