Steps to reproduce:
1. Open the attached project (ImportPackage Test.zip)
2. Open Assets/Editor/ImportPackageTest.cs and comment line 7 ([DidReloadScripts])
3. Return to Unity and allow scripts to reload
4. Open Assets/Editor/ImportPackageTest.cs and uncomment line 7 ([DidReloadScripts])
5. Return to Unity and allow scripts to reload
5. Observe the crash

Expected result: Unity should not crash
Actual result: Unity crashes

Note:
Issue does not occur with the Mac editor of the same version

Reproduced with: 5.5.0b8, 5.5.0f3, 5.5.0p3, 5.5.0p4, 5.6.0b3
Not reproduced with: 5.3.7p3, 5.4.3p4, 5.4.4p1, 5.5.0b7
Regression introduced in 5.5.0b8