Why can't we call a mocked method on a class variable?

I've been learning Scala recently, and right now, I am trying to do some learning tests with Mockito, but I've been facing something quite odd.

FooService.scala

class FooServie extends Serializable {

  val service:FooDaoTrait = FooDao
  val fooResult = service.getAlltheFoosFromDatabase     

  def putTheFoosInASet(): Set[String] = {
    FooResult.split(",").toSet
  }
}

FooServiceTest.scala

@RunWith(classOf[JUnitRunner])
class FooServiceTest extends FunSuite with MockitoSugar {

  val m = mock[FooDaoTrait]
  when(m.getAllTheFoosFromDatabase).thenReturn("Hi, my name is foo")

  val instance = new FooService{ override val service = m }

  test("Let's get da Foos") {
    assert(instance.putTheFoosInASet().size === 3) // 3 is a random Value
  }
}

So here, when i run the test, Scala IDE is throwing the one and only java.lang.NullPointerException on my class variable FooResult.

But Somehow, when i move FooResult inside the putTheFoosInASet():

  def putTheFoosInASet(): Set[String] = {
    val FooResult = service.getAlltheFoosFromDatabase  
    FooResult.split(",").toSet
  }

No exception is thrown and everything works well... Why does the previous pattern is throwing me an exception? I am not really sure what can cause this error and calling fooResult on different methods is not really clean imo.

Any opinion will be appreciated. Thanks.