Reproduction steps:
1. Download "Test.zip" project and open in Unity
2. Put any Tile from Tiles folder to Tile Palette
3. Draw in the Scene view
4. Close the project (select "Don't save")

Expected: should close properly
Actual: Fatal error pops up

Reproduced on: 2017.3.1f1; 2017.3.2f1; 2017.4.0f1; 2018.1.0b13; 2018.2.0a6

Verified with: 2018.3.0b4

Note:
This crash happens sometimes, but not always
Fatal error message:
"GetManagerFromContext: pointer to object of manager 'MonoManager' is NULL (table index 5)"
2017.2.0b1-2017.2.2p2, 2017.3.0a1-2017.3.1f1 crashes on startup or then opening Tile Palette with stack trace:
0x0000000141A8D479 (Unity) ValidateAndUpdateTileAssetDirty
0x0000000141AB1238 (Unity) Tilemap::Update
0x00000001403A10D6 (Unity) BaseBehaviourManager::CommonUpdate<BehaviourManager>
0x00000001403A1DEF (Unity) BehaviourManager::Update
0x00000001405D7323 (Unity) `InitPlayerLoopCallbacks'::`38'::UpdateScriptRunBehaviourUpdateRegistrator::Forward
0x00000001405D5A88 (Unity) PlayerLoop
0x0000000140F921AE (Unity) Application::UpdateScene
0x0000000140F945DF (Unity) Application::UpdateSceneIfNeeded
0x0000000140F9F701 (Unity) Application::TickTimer
0x00000001412B47CD (Unity) MainMessageLoop
0x00000001412B60DE (Unity) WinMain
0x0000000141DBF5F0 (Unity) __tmainCRTStartup
0x00007FFBCC771FE4 (KERNEL32) BaseThreadInitThunk
0x00007FFBCDC0EFC1 (ntdll) RtlUserThreadStart