Sorry- I don't have much info on this one. Hopefully the stacktrace will say more about it than I can. Posting so we have tracking, will update if I see it some more.

Here's what happened:
1. I had one instance of Unity open with a project loaded, I'd been working on this for a while.
2. I opened a new instance of Unity, when the launcher showed up the first instance crashed

The project I was working on (the instance that crashed) was using Mono 4.6

Unity 2017.3.0b8

========== OUTPUTING STACK TRACE ==================

0x00007FFD8C34CE95 (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\mini\aot-runtime.c:2479] mono_aot_get_cached_class_info
0x00007FFD8C4AC6DF (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\class.c:4890] mono_class_init
0x00007FFD8C4A5BC2 (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\class.c:2945] collect_implemented_interfaces_aux
0x00007FFD8C4B158F (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\class.c:4095] mono_class_setup_vtable_general
0x00007FFD8C4B14AE (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\class.c:3630] mono_class_setup_vtable_full
0x00007FFD8C4AB7BB (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\class.c:2540] mono_class_get_vtable_entry
0x00007FFD8C352779 (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\mini\mini-trampolines.c:903] mono_vcall_trampoline
ERROR: SymGetSymFromAddr64, GetLastError: 'The specified module could not be found.' (Address: 0000000011421838)
ERROR: SymGetModuleInfo64, GetLastError: 'A dynamic link library (DLL) initialization routine failed.' (Address: 0000000011421838)
0x0000000011421838 ((<unknown>)) (function-name not available)
0x0000000054FD771C (Mono JIT Code) System.Collections.Generic.Stack`1<T_REF>:Push (T_REF)
0x0000000054FD7390 (Mono JIT Code) [C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\Utility\ObjectPool.cs:46] UnityEngine.UI.ObjectPool`1<T_REF>:Release (T_REF)
0x0000000054FD701D (Mono JIT Code) [C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\Utility\ListPool.cs:20] UnityEngine.UI.ListPool`1<T_REF>:Release (System.Collections.Generic.List`1<T_REF>)
0x0000000054FEA6A7 (Mono JIT Code) [C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\MaskUtilities.cs:144] UnityEngine.UI.MaskUtilities:GetRectMaskForClippable (UnityEngine.UI.IClippable)
0x0000000054FD11E3 (Mono JIT Code) [C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\MaskableGraphic.cs:202] UnityEngine.UI.MaskableGraphic:UpdateClipParent ()
0x0000000054FD06EE (Mono JIT Code) [C:\buildslave\unity\build\Extensions\guisystem\UnityEngine.UI\UI\Core\MaskableGraphic.cs:150] UnityEngine.UI.MaskableGraphic:OnValidate ()
0x0000000054E0CF24 (Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)
0x00007FFD8C2B437A (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\mini\mini-runtime.c:2620] mono_jit_runtime_invoke
0x00007FFD8C4B8E87 (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\object.c:2853] do_runtime_invoke
0x00007FFD8C4C188F (mono-2.0-bdwgc) [c:\buildslave\mono\build\mono\metadata\object.c:2905] mono_runtime_invoke
0x0000000140A2BC4C (Unity) scripting_method_invoke
0x0000000140A1F03A (Unity) ScriptingInvocation::Invoke
0x0000000140A200AE (Unity) ScriptingInvocation::InvokeChecked
0x00000001409E1AF1 (Unity) MonoBehaviour::CallMethodInactive
0x00000001409EAB86 (Unity) MonoBehaviour::CheckConsistency
0x0000000140A081B8 (Unity) MonoManager::EndReloadAssembly
0x0000000140A088C5 (Unity) MonoManager::ReloadAssembly
0x0000000140C9A337 (Unity) ReloadAllUsedAssemblies
0x00000001411E777E (Unity) Application::TickTimer
0x000000014140C93F (Unity) MainMessageLoop
0x000000014140E1E5 (Unity) WinMain
0x0000000141E5B3D8 (Unity) __tmainCRTStartup
0x00007FFDCD492774 (KERNEL32) BaseThreadInitThunk
0x00007FFDCFF80D51 (ntdll) RtlUserThreadStart

========== END OF STACKTRACE ===========