To reproduce:
1. Open attached project "CancelFBXimport.zip"
2. Try building the project
3. Cancel the build when editor starts compiling shader variants or optimizing meshes

Result editor crashes

Reproduced in: 2019.1.10f1, 2019.2.0b9, 2019.3.0a5, 2019.3.0a7
Not reproduced in: 2017.4.30f1, 2018.4.4f1

Note1: no longerreproducible on 2019.3.0a8, which presents new issue - Cancel button is no longer usable, despite indicating it being active
Note2: not marked as regression due to issue appearing after implementing the Mesh Optimization feature

Crash stack trace:

0x00007FF65A442EA9 (Unity) CompileGpuProgramSnippets
0x00007FF65A444442 (Unity) CompileShaderSubprograms
0x00007FF65A443C5B (Unity) CompileShaderSubprograms
0x00007FF65CF397B9 (Unity) Shader::Transfer<StreamedBinaryWrite>
0x00007FF65ABE5ACD (Unity) SerializedFile::WriteObject
0x00007FF65ABB5841 (Unity) PersistentManager::WriteFile
0x00007FF65ABB6894 (Unity) PersistentManager::WriteFile
0x00007FF65A54EDDF (Unity) WriteSharedAssetFile
0x00007FF65A4E9AB6 (Unity) BuildExtraResourcesBundleForPlayer
0x00007FF65A51D287 (Unity) BuildPlayerData
0x00007FF65A52AF4C (Unity) DoBuildPlayer_Build
0x00007FF65A51A1D5 (Unity) BuildPlayer
0x00007FF65B1B3E9E (Unity) BuildPipeline::BuildPlayerInternalNoCheck
0x00007FF65B1B1944 (Unity) BuildPipeline_CUSTOM_BuildPlayerInternalNoCheck
0x00000184C0277083 (Mono JIT Code) (wrapper managed-to-native) UnityEditor.BuildPipeline:BuildPlayerInternalNoCheck (string[],string,string,UnityEditor.BuildTargetGroup,UnityEditor.BuildTarget,UnityEditor.BuildOptions,bool)
0x00000184C0276473 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:169] UnityEditor.BuildPlayerWindow/DefaultBuildMethods:BuildPlayer (UnityEditor.BuildPlayerOptions)
0x00000184BE26CFF3 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:96] UnityEditor.BuildPlayerWindow:CallBuildMethods (bool,UnityEditor.BuildOptions)
0x00000184BE255A1B (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindow.cs:900] UnityEditor.BuildPlayerWindow:GUIBuildButtons (UnityEditor.Modules.IBuildWindowExtension,bool,bool,bool,UnityEditor.Build.BuildPlatform)
0x00000184BE23F2FB (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindow.cs:791] UnityEditor.BuildPlayerWindow:ShowBuildTargetSettings ()
0x00000184BE225793 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindow.cs:359] UnityEditor.BuildPlayerWindow:OnGUI ()
0x00000184B1500700 (Mono JIT Code) (wrapper runtime-invoke) object:runtime_invoke_void__this__ (object,intptr,intptr,intptr)
0x00007FF9DF70BC80 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\mini\mini-runtime.c:2809] mono_jit_runtime_invoke
0x00007FF9DF691D92 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:2921] do_runtime_invoke
0x00007FF9DF69AF62 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:3073] mono_runtime_invoke_checked
0x00007FF9DF69B6F9 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:5264] mono_runtime_try_invoke_array
0x00007FF9DF69AEF6 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:5142] mono_runtime_invoke_array_checked
0x00007FF9DF63FF24 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\icall.c:3358] ves_icall_InternalInvoke
0x00000184B999B3A6 (Mono JIT Code) (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
0x00000184B999A0BB (Mono JIT Code) System.Reflection.MonoMethod:Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo)
0x00000184B9999D7F (Mono JIT Code) System.Reflection.MethodBase:Invoke (object,object[])
0x00000184B9990613 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\HostView.cs:337] UnityEditor.HostView:Invoke (string,object)
0x00000184B9990463 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\HostView.cs:332] UnityEditor.HostView:Invoke (string)
0x00000184BE2235E3 (Mono JIT Code) [C:\buildslave\unity\build\Editor\Mono\HostView.cs:138] UnityEditor.HostView:OldOnGUI ()
0x000001849644BA51 (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\IMGUIContainer.cs:281] UnityEngine.UIElements.IMGUIContainer:DoOnGUI (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect,bool,UnityEngine.Rect)
0x0000018496449A3B (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\IMGUIContainer.cs:485] UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect)
0x00000184BD09028B (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\IMGUIContainer.cs:466] UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event)
0x00000184B9988793 (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\IMGUIContainer.cs:447] UnityEngine.UIElements.IMGUIContainer:HandleEvent (UnityEngine.UIElements.EventBase)
0x00000184BD089320 (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\Events\MouseCaptureDispatchingStrategy.cs:98] UnityEngine.UIElements.MouseCaptureDispatchingStrategy:DispatchEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel)
0x00000184B99493DF (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\EventDispatcher.cs:282] UnityEngine.UIElements.EventDispatcher:ProcessEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel)
0x00000184B9943413 (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\EventDispatcher.cs:157] UnityEngine.UIElements.EventDispatcher:Dispatch (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel,UnityEngine.UIElements.DispatchMode)
0x00000184B9943033 (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\Panel.cs:191] UnityEngine.UIElements.BaseVisualElementPanel:SendEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.DispatchMode)
0x00000184B9A6841B (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\UIElementsUtility.cs:258] UnityEngine.UIElements.UIElementsUtility:DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel)
0x00000184B9A67B4B (Mono JIT Code) [C:\buildslave\unity\build\Modules\UIElements\UIElementsUtility.cs:78] UnityEngine.UIElements.UIElementsUtility:ProcessEvent (int,intptr)
0x00000184B9A674AE (Mono JIT Code) [C:\buildslave\unity\build\Modules\IMGUI\GUIUtility.cs:179] UnityEngine.GUIUtility:ProcessEvent (int,intptr)
0x00000184B9A676D3 (Mono JIT Code) (wrapper runtime-invoke) <Module>:runtime_invoke_bool_int_intptr (object,intptr,intptr,intptr)
0x00007FF9DF70BC80 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\mini\mini-runtime.c:2809] mono_jit_runtime_invoke
0x00007FF9DF691D92 (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:2921] do_runtime_invoke
0x00007FF9DF69AD8F (mono-2.0-bdwgc) [c:\users\bokken\builds\vm\mono\mono\metadata\object.c:2968] mono_runtime_invoke
0x00007FF65C302C22 (Unity) scripting_method_invoke
0x00007FF65C2FC8E1 (Unity) ScriptingInvocation::Invoke
0x00007FF65C2F6B3A (Unity) ScriptingInvocation::Invoke<bool>
0x00007FF65987B499 (Unity) Scripting::UnityEngine::GUIUtilityProxy::ProcessEvent
0x00007FF65A2D7EDC (Unity) GUIView::ProcessRetainedMode
0x00007FF65AC49DF7 (Unity) GUIView::OnInputEvent
0x00007FF65A2D7DEC (Unity) GUIView::ProcessInputEvent
0x00007FF65AC4B705 (Unity) GUIView::ProcessEventMessages
0x00007FF65AC3F93B (Unity) GUIView::GUIViewWndProc
0x00007FFA2E096D41 (USER32) CallWindowProcW
0x00007FFA2E096713 (USER32) DispatchMessageW
0x00007FF65AC2E066 (Unity) MainMessageLoop
0x00007FF65AC308FF (Unity) WinMain
0x00007FF65D8B8B62 (Unity) __scrt_common_main_seh
0x00007FFA307D4034 (KERNEL32) BaseThreadInitThunk
0x00007FFA30923691 (ntdll) RtlUserThreadStart