DoSomething returns null instead of returning a Task, and so you get an exception when awaiting it. You need to specify when building the mock that it should return a Task.
In this case it seems that you can simply return an already completed task using Task.FromResult so the mock setup should look like this:
this._mockService.Setup(...).Returns(Task.FromResult(false));
Beginning with the next version of .Net (4.6) you can use Task.CompletedTask like this:
this._mockService.Setup(...).Returns(Task.CompletedTask);