How to reproduce:
1. Open the attached project
2. Selected two or more Scriptable Objects from "Assets/MyTiles"
Optional steps:
3. Move the cursor over the Inspector window
4. Open the Tile Palette

Expected result: These actions do not result in NullReferenceExceptions or the errors have user-friendly descriptions.
Actual result: The upper-mentioned actions result in multiple NullReferenceExcetion errors (in the Console).

Reproducible with - 2017.3.0p2, 2017.4.5f1, 2018.1.0b1, 2018.1.5f1, 2018.2.0b8, 2018.3.0a1
Not reproducible with - 2017.1.4p1, 2017.2.3p1, 2017.3.0p1, 2018.1.0a7

The error message:
NullReferenceException
UnityEditor.InspectorWindow.HasLabel (UnityEngine.Object target, System.String assetPath) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:715)
UnityEditor.InspectorWindow.HasLabel (UnityEngine.Object target) (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:710)
System.Linq.Enumerable+<CreateWhereIterator>c__Iterator1D`1[UnityEngine.Object].MoveNext ()
System.Collections.Generic.List`1[UnityEngine.Object].AddEnumerable (IEnumerable`1 enumerable) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections.Generic/List.cs:128)
...

Backported to: 2018.2.0f1