System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.20

Blender Version
Broken: version: 2.90.0 Alpha, branch: master, commit date: 2020-06-04 22:58, hash: rBc7329da14b22
Worked: 2.90 Alpha 2020 - 06-02
Short description of error
When using custom modal operator, auto save causes crash:
The custom modal {F8584443}operator does not crash when auto save disabled.
Crash Log:
Info: Saved "Waneella_33.blend"
Skipping auto-save, modal operator running, retrying in ten seconds...
Memoryblock free: pointer not in memlist
Info: Saved "Waneella_33.blend"
Memoryblock free: pointer not in memlist
Memoryblock free: pointer not in memlist
Info: Saved "Waneella_33.blend"
Memoryblock free: pointer not in memlist
Memoryblock free: pointer not in memlist
running modal
Scaling
Applying Scale
Memoryblock free: pointer not in memlist
running modal
Scaling
Memoryblock Chunk buffer: double free
Memoryblock free: pointer not in memlist
Memoryblock free: pointer not in memlist
Memoryblock Chunk buffer: double free
Memoryblock Chunk buffer: double free

Exact steps for others to reproduce the error
custom modal operator is attached, if you are in modal mode while autosave happens, the Memoryblock errors start showing. Does not crash on first autosave but eventually will crash with too many memoryblock errors.
[Based on the default startup or an attached .blend file (as simple as possible)]