How to reproduce:
1. Open the attached "1101390_repro.zip" project
2. In Project window, delete "Packages/2D Common" package folder
3. Also, delete these three files one by one:

• Packages/2D Animation/Runtime/Unity.2D.Animat.Runtime.asmdef
• Packages/2D Animation/Tests/Editor/Unity.2D.Animat.EditorTests.asmdef
• Packages/2D Animation/Editor/Unity.2D.Animat.Editor.asmdef

4. Wait for the recompiling process to finish
-- Editor crash

Reproducible with - 2018.3.0a10, 2018.3.0b11, 2019.1.0a1, 2019.1.0a9
Not reproducible with - 2018.2.17f1, 2018.3.0a9

Note: Could not be tested on versions prior to 2018.2, since the above mentioned "Packages" folder was introduced in 2018.2.

Stack trace:
(Unity) scripting_array_length
(Unity) ToDynamicArrayOfTargetAssemblyInfo
(Unity) GetTargetAssembliesWithScripts
(Unity) MonoManager::AreUserAssembliesReady
(Unity) ShaderImporter::OnAssembliesLoaded
(Unity) CallbackArray::Invoke
(Unity) MonoManager::EndReloadAssembly
(Unity) MonoManager::ReloadAssembly
(Unity) ReloadAllUsedAssemblies
(Unity) Application::TickTimer
(Unity) MainMessageLoop
(Unity) WinMain
(Unity) __scrt_common_main_seh
(KERNEL32) BaseThreadInitThunk
(ntdll) RtlUserThreadStart

Fixed in 2019.1.0a11
Backported to 2018.3.1f1