Steps to repro:
1. Open attached project;
2. In the project folder go to 'Free_SpeedTrees' folder;
3. Multi-select 'Broadleaf-Desktop.spm' tree prefab with other assets in the scene by using shift+ left mouse click.

Actual result:

Error:

'NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header, Single leftMargin) (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:486)
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header) (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:465)
UnityEditor.InspectorWindow.DrawSelectionPickerList () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1328)
UnityEditor.InspectorWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:366)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)'

is thrown in the console. See attached video.
Regression introduced in Unity 2017.1.0b1.

Expected result:
Errors should not be thrown when multi-selecting speedtree prefab with other project assets.

Notes:
- Reproducible in 2017.1.0b1;
- Not reproducible in 2017.1.0a6, 5.6.0p1, 5.6.0f3, 5.5.2p2;
- Reproducible on Windows 10 and OSX 10.12.3;
- The bug is reproducible only when speedtree prefab is selected; Not reproducible for multiselecting regular prefab with other assets of just several simple assets (textures, sprites, etc).