Steps to reproduce:

1. Open Unity;
2. Open attached project and ‘test.unity’ scene;
3. In the ‘Project’ window go to Assets —> Free_SpeedTrees folder;
4. Select any .spm model (for instance, ‘Broadleaf_Mobile.spm’) and drag it to the SceneView;
5. Delete recently added .spm file from the ‘Project’ folder;
6. In the Hierarchy tab expand deleted ‘Broadleaf_Mobile’ (it should be coloured in red);
7. Either click on ‘Broadleaf_Mobile_Billboard’ object or scroll out in the Scene View so that Billboard LOD level of recently deleted model would become "visible".

Actual result:
Unity crashes in BillboardAsset::AcquireBillboardData() thread
(see attached gif video).

Last calls in the stacktace:
#0 0x00000100f01b39 in BillboardAsset::AcquireBillboardData() const
#1 0x00000100f05953 in BillboardRenderer::AddAsRenderNode(RenderNodeQueue&, DeprecatedSourceData const&)
#2 0x00000100d012fe in PrepareRenderNodesOnMainThreadDeprecated(RenderNodeQueuePrepareThreadContext&)
#3 0x00000100d005c9 in EndRenderQueueExtraction(RenderNodeQueuePrepareContext*)
#4 0x00000100c7a78c in CreateSharedRendererScene(CullResults const&)
#5 0x000001019cfc68 in Camera::RenderEditorCamera(Camera::EditorDrawingMode, DrawGridParameters const*)

Regression introduced in Unity 5.4.0.

Expected result:
Unity should not crash when picking deleted Billboard LOD level in the Editor.

Notes:
- Reproducible on both OSX 10.11.3 and Windows 10;
- Reproducible in 5.4.0b7, 5.4.0b5, 5.4.0b4, 5.4.0a4;
- Not reproducible in 5.3.1f1.