Steps to repro:

1. Open attached project and 'test' scene;
2. Select Terrain gameobject in the 'Hierarchy';
3. Go to 'Paint Texture' tab;
4. Click Edit Textures -> Add Textures;
5. Observe the console.

Actual result:

Error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TerrainSplatEditor.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainSplatEditor.cs:201)

is thrown. See attached gif video.

Regression introduced in 5.4.0b22.

Same behaviour happens when Opening 'Add Tree' window from 'Paint Trees' tab, however the error in this case is slightly different:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.TreeWizard.DrawWizardGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/TerrainEditor/TerrainWizards.cs:412)
UnityEditor.ScriptableWizard.OnGUI () (at /Users/builduser/buildslave/unity/build/artifacts/generated/common/editor/

See attached screenshots.

Expected result:
Error should not be thrown when opening Add Texture/Tree popup windows.

Notes:
- Reproducible in 5.4.0b22;
- Reproducible only on OSX 10.11.4;
- Not reproducible on Windows 10;
- The issue seems not to affect abovementioned functionality. Texture/Tree assigning works as expected.