Steps to repro:

1. Open Unity;
2. Create new project;
3. Extract attached package and open 'testPrefabs' scene;
4. In the Project folder go to Assets -> Free_SpeedTrees;
5. Select 'Broadleaf_Desktop' .spm model;
6. In the inspector input some value to Scale Factor component (e.g. 1);
7. Hit 'Apply Prefab'.

Actual result:
Editor crashes in OSAtomicAdd32Barrier$VARIANT$mp (see attached gif video). Last calls in the stacktrace:

#0 0x007fff86fe79f6 in OSAtomicAdd32Barrier$VARIANT$mp
#1 0x00000101f252d2 in LODTreeInstanceRenderer::AddAsRenderNode(RenderNodeQueue&, DeprecatedSourceData const&)
#2 0x000001016b5eba in PrepareRenderNodesOnMainThreadDeprecated(RenderNodeQueuePrepareThreadContext&)
#3 0x000001016a142d in EndRenderQueueExtraction(RenderNodeQueuePrepareContext*)
#4 0x0000010162069f in CullResults::GetOrCreateSharedRendererScene()
#5 0x0000010162020e in Camera::Render(CullResults&, ShaderPassContext&, CameraRenderingParams const*, Camera::RenderFlag, void*)

Expected result:
- Editor should not crash;
- Scale factor should be updated for painted prefabs created from SpeedTree model.

Notes:
- Reproduces on: 5.5.0b4, 5.5.0b3, 5.4.1p1, 5.4.0f3, 5.3.5f1;
- Reproduces on both OSX 10.11.6 and Windows 10;
- Not reproducible when toggling scale of painted .spm model; happening only for prefabs created from .spm models.