How to reproduce:
1. Open the user-submitted project ("NavMeshComponents.zip")
2. Click on the "Link" GameObject
3. Disable the "NavMeshLink" component and deselect the object
4. Click on the Navigation Window and observe the navigation mesh in the Scene View
5. Enter and leave the Play Mode
6. Observe the navigation mesh in the Scene View again
7. Click on the "Link" GameObject again and observe the navmesh in the scene
8. Try to re-enable the "NavMeshLink" component

Expected result: The "NavMeshLink" component is immediately disabled and the mesh link is not visible in the scene
Actual result: The Nav Mesh Link is reintroduced into the Scene when clicking on the "Link" GameObject and trying to re-enable the component throws the "Link is already added: Link (UnityEngine.AI.NavMeshLink) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)" error

Reproduced in: 2019.2.0a6, 2019.1.0b4, 2018.3.7f1, 2018.3.0b1
Couldn't test with: 2018.3.0a11, 2017.4.21f1 (due to multiple project errors when downgrading)