Mockito verify that variable is not null

In my class FooController I have a public variable that is set to null

var foo: Foo? = null

and a public void method to init the variable

fun initFoo() { foo = Foo() } In my test I mock the class FooController and called the method initFoo

mockFooController.initFoo()

Now I want to verify that the variable foo is not null. How can I do this with Mockito?