Revert VersionedControllerSelector.cs from commit 4d71fd30f6800e374000e551421c30ff0e5c2f05.
Wanted this as a separate commit.
Only throw exception if a new value is being set.
Example scenario: In unit testing, the static value may be set multiple times when running multiple tests. This will cause all but the first test to fail on this exception unless the test explicitly tests the value before assigning, which I think is unnecessary to require.