TestStack.White "freezes" after launching another application

I'm new in testing UI with TestStack.White library. And I stucked with the next problem.

1. Main and child applications with WPF controls
2. OPEN button in the main app calls the child app
3. After event Button.Click() on button OPEN child app appears, and nothing happens.
4. Any statement cannot be performed until child app will be closed (it can be clearly seen during debugging).
5. I can neither get title of new window, nor get any control state, nor write something to trace.
6. Child application can be launched separately, and all controls are available

Who is faced with such problem?