Using 2018.3.3f1 and still getting this error.

Invalid editor window UnityEditor.FallbackEditorWindow
0x0000000141415408 (Unity) StackWalker::GetCurrentCallstack
0x000000014141B8B6 (Unity) StackWalker::ShowCallstack
0x00000001412FF13B (Unity) GetStacktrace
0x00000001401432FE (Unity) DebugStringToFile
0x00000001417868DD (Unity) DebugLogHandler_CUSTOM_Internal_Log
0x000000001DD291B0 (Mono JIT Code) (wrapper managed-to-native) UnityEngine.DebugLogHandler:Internal_Log (UnityEngine.LogType,string,UnityEngine.Object)
0x000000001DD28E13 (Mono JIT Code) [DebugLogHandler.cs:10] UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
0x000000001DD2868D (Mono JIT Code) [Logger.cs:48] UnityEngine.Logger:Log (UnityEngine.LogType,object)
0x0000000045BE1D59 (Mono JIT Code) [Debug.bindings.cs:117] UnityEngine.Debug:LogError (object)
0x0000000045BE1B8B (Mono JIT Code) [WindowLayout.cs:54] UnityEditor.WindowLayout:CheckWindowConsistency ()
0x0000000045BE1343 (Mono JIT Code) [WindowLayout.cs:465] UnityEditor.WindowLayout:MaximizePresent (UnityEditor.EditorWindow)
0x0000000045BE111B (Mono JIT Code) [EditorApplicationLayout.cs:77] UnityEditor.EditorApplicationLayout:FinalizePlaymodeLayout ()
0x0000000055D8B0B5 (Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void (object,intptr,intptr,intptr)
0x00007FFD40EFBE4B (mono-2.0-bdwgc) [mini-runtime.c:2809] mono_jit_runtime_invoke
0x00007FFD40E81E32 (mono-2.0-bdwgc) [object.c:2919] do_runtime_invoke
0x00007FFD40E8AE3F (mono-2.0-bdwgc) [object.c:2966] mono_runtime_invoke
0x0000000140BEF80B (Unity) CallStaticMonoMethod
0x0000000140BEF59B (Unity) CallStaticMonoMethod
0x000000014132D37B (Unity) PlayerLoopController::EnterPlayMode
0x00000001413392D3 (Unity) PlayerLoopController::SetIsPlaying
0x000000014133C1B2 (Unity) Application::TickTimer
0x0000000141497DAB (Unity) MainMessageLoop
0x0000000141499A46 (Unity) WinMain
0x000000014247CC4A (Unity) __scrt_common_main_seh
0x00007FFDAE383DC4 (KERNEL32) BaseThreadInitThunk
0x00007FFDB0B03691 (ntdll) RtlUserThreadStart

I have this now on 2018.3.6f1 it says fixed but how?

Also happens on latest 2018.2.10f1

Assertion failed on expression: 'm_PreloadTextureAtlasData.size() == m_DetailPrototypes.size()'
UnityEngine.Terrain:get_terrainData()
VEAGames.LAM.LAMTerrainTool:OnValidate() (at Assets/Plugins/LandscapeAutoMaterial/Scripts/LAMTerrainTool.cs:105)

Also happens on 5.6.4f1.

Console:
Assertion failed on expression: 'm_PreloadTextureAtlasData.size() == m_DetailPrototypes.size()'
UnityEditor.HostView:OnGUI()

Still happens on 2017.4.3f1 and most likely happens on any active Unity version.

Also happens here on 2017.1.2p2

I have that issue as well. I don't know where it's coming from, except that it appears only when I change scene or save it. Then, it just break details objects on that corrupted map. I have to double click on one of them in the terrain inspector, so it come back to his original form (Details tab). I tried on a new terrain and it doesn't seem to appear for now, only on a old map that I imported over and over, when I switched to another version of Unity... Maybe that old map is corrupted or something?

Here what I get when I use the Stack Trace Logging (Full) :

Assertion failed: Assertion failed on expression: 'm_PreloadTextureAtlasData.size() == m_DetailPrototypes.size()'
0x00000001412F21A6 (Unity) StackWalker::GetCurrentCallstack
0x00000001412F2D1F (Unity) StackWalker::ShowCallstack
0x00000001411B0910 (Unity) GetStacktrace
0x00000001406DAF53 (Unity) DebugStringToFile
0x00000001406DB731 (Unity) DebugStringToFile
0x0000000141A82BC4 (Unity) DetailDatabase::GenerateTextureAtlasThreaded
0x0000000140A9325D (Unity) PersistentManager::PostReadActivationQueue
0x0000000140A9E9F2 (Unity) PersistentManager::ReadAndActivateObjectThreaded
0x0000000140A9ED69 (Unity) PersistentManager::LoadRemainingPreallocatedObjectsInternal
0x0000000140AA0C9D (Unity) PersistentManager::LoadFileCompletelyThreaded
0x00000001408002A3 (Unity) LoadSceneOperation::Perform
0x00000001408007B3 (Unity) PreloadManager::ProcessSingleOperation
0x00000001408008FB (Unity) PreloadManager::Run
0x0000000140800C8E (Unity) PreloadManager::Run
0x00000001408B78E8 (Unity) Thread::RunThreadWrapper
0x000000007788652D (kernel32) BaseThreadInitThunk
0x00000000779BC541 (ntdll) RtlUserThreadStart

I hope it gets fixed soon, I can't do anything on this map because of that error, can't even place details...Thanks ! :)

Yes, I have same issue here with Terrain and CTS. Cant paint correctly anymore because of that :/

Also happens here on latest 2017.3.0p1