Unit test: checking for a NPE where exists an error before

I am developing a plugin for PhpStorm, and in some point, it try to method.getContainingClass(). This method should return NULL when the class name is absent. Just to make easier to explain, my code is like that:

final PhpClass methodClass = method.getContainingClass();
assert methodClass != null;

Note that the assert is wrong here, just to make the code flow (my mistake here, because initially I thought that visitPhpMethod() is never called when the getContainingClass() could be null). And my problem is just to make this assert fail programmatically (from unit test). From real PhpStorm application, running the builded plugin, I can do that fails when I just remove the class name like that:

<?php
class { function x(){} }

// ~> java.lang.AssertionError (failed that methodClass != null)

But when I run the same code from unit test, it give that:

<?php
class<error descr="Expected: class name"> </error>{ function x(){} }

And with that, the visitPhpMethod() is not triggered (while in real PhpStorm application it does). In this way, is impossible to do a proof case, which is needly to I check if I can keep this assert or not (for instance).