Repro steps (original attached project):
1. Drag "MissilesCollision" prefab into the scene.
2. Make sure the Particle Editor is open
3. In the Particle Editor, expand the Collisions module and change the type to Planes

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I've also attached a smaller project, Bug_01.rar. If you want to use this, the repro steps are:
1. Open Scene 1
2. Select the Particle System prefab instance in the scene, and open the Editor.
3. Change the Collisions module's type to Planes.

Observe error / UI messes up like in the screenshot above.

The following errors appear in the console:

NullReferenceException: Object reference not set to an instance of an object
UnityEditorInternal.EditMode.DoInspectorToolbar (.SceneViewEditMode[] modes, UnityEngine.GUIContent[] guiContents, Bounds bounds, UnityEditor.Editor caller) (at C:/buildslave/unity/build/Editor/Mono/Inspector/EditMode.cs:190)
UnityEditor.ModuleUI.GUIButtonGroup (.SceneViewEditMode[] modes, UnityEngine.GUIContent[] guiContents, Bounds bounds, UnityEditor.Editor caller) (at C:/buildslave/unity/build/Editor/Mono/ParticleSystemEditor/ParticleSystemModules/ModuleUIHelpers.cs:159)
UnityEditor.CollisionModuleUI.OnInspectorGUI (UnityEditor.InitialModuleUI initial) (at C:/buildslave/unity/build/Editor/Mono/ParticleSystemEditor/ParticleSystemModules/CollisionModuleUI.cs:230)
UnityEditor.ParticleSystemUI.OnGUI (Single width, Boolean fixedWidth) (at C:/buildslave/unity/build/Editor/Mono/ParticleSystemEditor/ParticleSystemUI.cs:190)

Notes:
- Not a regression, happens in 2017.1 and 2017.2