Guard MethodInfoSelector(Assertions) against null reference exceptions

[eNeRGy164]

More coverage of #1039

Follows the changes made in #1565

• Added failure message when Subject is null
• Added Guards against null parameters

Will need a rebase after #1565 is merged.

IMPORTANT

• The code complies with the Coding Guidelines for C#.
• The changes are covered by a new or existing set of unit tests which follow the Arrange-Act-Assert syntax such as is used in this example.
• If the contribution adds a feature or fixes a bug, please update the release notes, which are published on the website.