Steps to reproduce:
1. Create a new project A
2. Create a new project B
3. Place B into A's Assets folder
4. Attempt to open A

Actual result: Unity crashes without any error
Expected result: Unity should produce a Fatal Error window that was produced in previous versions

Reproducible with: 2020.1.0a16, 2020.1.0b14, 2020.2.0a16
Not reproducible with: 2018.4.24f1, 2019.3.16f1, 2019.4.3f1, 2020.1.0a15

First lines of the stacktrace:
0x00007FF7AB93D3D3 (Unity) SceneVisibilityState::GetHiddenObjectCount
0x00007FF7AC30AF46 (Unity) SceneVisibilityState_CUSTOM_GetHiddenObjectCount
0x00000258AA5BB0C1 (Mono JIT Code) (wrapper managed-to-native) UnityEditor.SceneVisibilityState:GetHiddenObjectCount ()
0x00000258AA5BB003 (Mono JIT Code) UnityEditor.SceneView:UpdateHiddenObjectCount ()
0x00000258AA5B4723 (Mono JIT Code) UnityEditor.SceneView:OnEnable ()

Notes:
- In the versions deemed not reproducible instead of a crash there's a Fatal Error window warning the user that there's multiple Library folders within the project
- Before the crash there's this assertion failure in the logs:
"Assertion failed on expression: 'gSingleton != NULL'"