Interaction on null mock object shall throw exception

An interaction on a null object shall report an exception to the user, that the interaction does not make sense.
This shall help users to detect such interactions, which will work as intended.

This is a breaking change, because previously such interactions are silent, although probably senseless.

class TestSpec extends Specification {

  def "Interaction on null Object shall report error"() {
    given:
    Object nullObj = null

    when:
    nullObj .toString() >> ""

    then:
    def ex = thrown(IllegalArgumentException)
    ex.message == "Interaction on null object is invalid. TODO message"
  }
}

Start by locating the handling for the shown nullObj.toString() >> "" interaction and its current silent behavior. Verify the change with the provided TestSpec scenario: the interaction should throw IllegalArgumentException and report the message required by the issue.