"Create Extension Service" triggers exception when MixedRealityToolkit.Extensions is missing

[gilbdev]

Describe the bug

when i want to create a new extension service, i got the following exception.
I've put the "MixedRealityTookit.Tools" folder in my solution so i have access to the extension service wizard, but i didn't add the "MixedRealityToolkit.Extensions" folder and what's in it (.nuspec, .sentinel, License.txt, Version.txt)

apparently one of those files is required for the wizard to work properly.

NullReferenceException: Object reference not set to an instance of an object
Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitFiles.FormatSeparatorsForUnity (System.String path) (at Assets/MixedRealityToolkit/Utilities/Editor/Setup/MixedRealityToolkitFiles.cs:237)
Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitFiles.GetAssetDatabasePath (System.String absolutePath) (at Assets/MixedRealityToolkit/Utilities/Editor/Setup/MixedRealityToolkitFiles.cs:245)
Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitFiles.MapModulePath (Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitModuleType module) (at Assets/MixedRealityToolkit/Utilities/Editor/Setup/MixedRealityToolkitFiles.cs:341)
Microsoft.MixedReality.Toolkit.Editor.ExtensionServiceCreator.get_ExtensionsFolder () (at Assets/MixedRealityToolkit.Tools/ExtensionServiceCreator/ExtensionServiceCreator.cs:86)
Microsoft.MixedReality.Toolkit.Editor.ExtensionServiceCreator.ValidateAssets (System.Collections.Generic.List`1[T] errors) (at Assets/MixedRealityToolkit.Tools/ExtensionServiceCreator/ExtensionServiceCreator.cs:285)
Microsoft.MixedReality.Toolkit.Editor.ExtensionServiceWizard.OnGUI () (at Assets/MixedRealityToolkit.Tools/ExtensionServiceCreator/ExtensionServiceWizard.cs:65)

A clear and concise description of what the bug is.

To reproduce

Steps to reproduce the behavior:

1. in a project with MixedRealityToolkit.Tools but not MixedRealityToolkit.Extensions,
2. click in the menu MixedRealityToolkit > Utilities > Create Extension Service
   => observe an empty window opens and errors in log

Expected behavior

• At least a proper warning message saying that the extension package is missing in the project
  -or recreate needed files if possible

Your setup (please complete the following information)

• Unity Version [2018.4.13f1]
• MRTK Version [v2.2]