To reproduce:
1. Create new project
2. Open build settings
3. Switch to Facebook platform
4. Notice that Build settings window becomes unresponsive (window can be closed, but everything else is unresponsive) and Argument null exception is thrown

Exception:

ArgumentNullException: Argument cannot be null.
Parameter name: path1
System.IO.Path.Combine (System.String path1, System.String path2) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Path.cs:112)
UnityEditor.Facebook.SDKManager.get_FacebookUnityAssemblyPath () (at C:/buildslave/unity/build/PlatformDependent/Facebook/Extensions/Managed/SDKManager.cs:113)
UnityEditor.Facebook.FacebookSettingsExtension.GetFacebookSettingsType () (at C:/buildslave/unity/build/PlatformDependent/Facebook/Extensions/Managed/FacebookSettingsExtension.cs:101)
UnityEditor.Facebook.FacebookSettingsExtension.InitFacebookSettings () (at C:/buildslave/unity/build/PlatformDependent/Facebook/Extensions/Managed/FacebookSettingsExtension.cs:43)
UnityEditor.Facebook.FacebookSettingsExtension.get_AppID () (at C:/buildslave/unity/build/PlatformDependent/Facebook/Extensions/Managed/FacebookSettingsExtension.cs:64)
UnityEditor.Facebook.BuildWindowExtension.ShowPlatformBuildOptions () (at C:/buildslave/unity/build/PlatformDependent/Facebook/Extensions/Managed/BuildWindowExtension.cs:158)
UnityEditor.BuildPlayerWindow.ShowBuildTargetSettings () (at C:/buildslave/unity/build/Editor/Mono/BuildPlayerWindow.cs:1190)
UnityEditor.BuildPlayerWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/BuildPlayerWindow.cs:909)
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)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
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:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:249)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:242)
UnityEditor.HostView.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:76)

Reproducible: 5.6.0a1