System Information
Operating system: Linux-5.2.8-200.fc30.x86_64-x86_64-with-fedora-30-Thirty 64 Bits
Graphics card: GeForce GTX 970M/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.26

Blender Version
Broken: version: 2.81 (sub 3), branch: master, commit date: 2019-08-23 02:03, hash: rB232049dd9408
Worked: I think before rBd357e7b06535

Short description of error
Assert entering editmode with a UVEditor open

Exact steps for others to reproduce the error

• launch blender --factory-startup
• split the 3DView
• change left Editor Type to UV Editor
• change to editmode in the 3DView
• move mouse back to UV Editor --> boom

BLI_assert failed: /blender/source/blender/draw/intern/draw_cache_extract_mesh.c:525, extract_tris_finish(), at 'mr->cache->surface_per_mat[0]->elem == ibo'