Steps to reproduce:
1. Download "mini.tgz" attached by the user
2. Enter "player" scene
3. Drag-and-drop "scene-0-0" and "scene-1-1" to hierarchy window
4. Right click on "scene-0-0" and press "Unload Scene"

Result: Crash
#0 0x007ffdc8dc81c0 in funlockfile
#1 0x007ffdc8dc8760 in Material::UnshareMaterialData()
#2 0x007ffdc8dc87c0 in Material::DisableKeyword(core::basic_string<char, core::StringStorageDefault<char> > const&)
#3 0x007ffdc8dc87e0 in UI::Canvas::UpdateRectClipping(UI::Batch&, Material*)
#4 0x007ffdc8dc8840 in UI::Canvas::DrawIntermediateRenderer(UI::Batch&, Matrix4x4f, int, Camera*, unsigned short)
#5 0x007ffdc8dc8900 in UI::Canvas::EmitWorldGeometry(Camera*, unsigned short&, bool)
<...>

Reproduced with: 2017.3.0b1 (Linux editor)
Did not reproduce with: 2017.1.1f1, 2017.2.0f3