public S getInstance() {
    if (theInstance == null) {
      throw new InstanceContextException();
    }
    return (S) theInstance;
  }
Returns the Test Instance
<p>
If accessed, the instance' setup will run before this can be evaluated.
@since 2.0
@return the test instance
spock-core/src/main/java/org/spockframework/runtime/extension/builtin/PreconditionContext.java