I having a Controller like

def testFunction(testCommand cmdObj) {
    if (cmdObj.hasErrors()) {
        render(view: "testView", model: [cmdObj:cmdObj])
        return
    } else {
        try {
            testService.testFunction(cmdObj.var1, cmdObj.var2, session.user.username as String)
            flash.message = message(code: 'message')
            redirect url: createLink(mapping: 'namedUrl')
        } catch (GeneralException error) {
            render(view: "testView", model: [cmdObj:cmdObj])
            return
        }
    }
}

For the above controller function I having a Unit test function like:

def "test function" () {
        controller.session.user.username = "testUser"
        def testCommandOj = new testCommand(
                var1:var1,
                var2:var2,
                var3:var3,
                var4:var4
        )
        testService service = Mock(testService)
        controller.testService = service
        service.testFunction(var2,var3,var4)

    when: 
        controller.testFunction(testCommandOj)

    then: 
        view == "testView"
        assertTrue model.cmdObj.hasErrors()

    where:
        var1            | var2      | var3      | var4
        "testuser"      |  "word@3" | "word@4"  | "word@4"

}

When running this test function I getting the error like Cannot set property 'username' on null object, means I couldn't able to set up the session object. Can someone help to fix this. Thanks in advance