1. What happened
NullReferenceException thrown when selecting multiple objects of different types in Project explorer:

NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header, Single leftMargin) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:486)
UnityEditor.Editor.DrawHeaderGUI (UnityEditor.Editor editor, System.String header) (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:465)
UnityEditor.InspectorWindow.DrawSelectionPickerList () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:1328)
UnityEditor.InspectorWindow.OnGUI () (at /Users/builduser/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)

2. How we can reproduce it using the example you attached
- Open Project Explorer
- Select few objects of different type (either Cmd + A for all objects, or e.g. Folder + Script). 2 objects are enough as longs as they're not the same type
--->
Error appears in Console and is constantly generated as long as objects are selected

This is regression from 5.6.0p1