NullPointerException in Grails unit tests because of null Params and Session - unit-testing

Question

When migrating from Grails 2.0.0 to 2.1.2 some of our tests started failing with NullPointerException (the same behavior is observed with Grails 2.0.3)
Here are snippets of code enough to reproduce the issue.
The controller:
class TestController {
def test() {
render(template: "/test")
}
}
The unit test:
import static org.junit.Assert.*
import grails.test.mixin.*
import grails.test.mixin.support.*
import org.junit.*
import grails.test.mixin.web.GroovyPageUnitTestMixin
#TestMixin(GroovyPageUnitTestMixin)
#TestFor(TestController)
class TestControllerTests {
void test_paramsAndSession_Null() {
controller.test()
def result = response.text
print result
assert render(template: "/test") == result
}
}
The template _test.gsp:
Params and session test.
Params(<%params.id1%>)<br/>
Session(<%session.id2%>)<br/>
With Grails 2.0.0 the application and the test work fine.
With Grails 2.1.2 the application still works fine, but the test starts failing with NullPointerException:
java.lang.NullPointerException: Cannot get property 'id1' on null object
at D__Eclipse_Workspace_paramsAndSessionTest_grails_app_views__test_gsp.run(_test.gsp:2)
at TestController.test(TestController.groovy:4)
at TestControllerTests.test_paramsAndSession_Null(TestControllerTests.groovy:12)
java.lang.NullPointerException: Cannot get property 'id2' on null object
at D__Eclipse_Workspace_paramsAndSessionTest_grails_app_views__test_gsp.run(_test.gsp:3)
at TestController.test(TestController.groovy:4)
at TestControllerTests.test_paramsAndSession_Null(TestControllerTests.groovy:12)
(The second exception appears when fix (1) below is applied)
What is the reason for that?
I see I can fix the test making the following changes:
params.id1 -> params?.id1 (1)
session.id2 -> session?.id2
But this does not look like a valid solution (changing my application for unit tests to work).
Please help me understand whether I am doing something wrong or there is a bug in Grails.
UPDATE: I've submitted a bug http://jira.grails.org/browse/GRAILS-9718