Actions.popTo("screenKeyName") not working, says undefined route name.

### Version
Tell us which versions you are using:

- react-native-router-flux v 4.0.6
- react v16.4.2
- react-native v0.55.4

### Expected behaviour
I have 2 tabs (TabA and TabB). When a user navigates from Tab A to B, some screen let say B-Main is open which list on which some navigation takes place consider the navigated screen to be B-subScreen.
When I press the back option **from B-subScreen** , I want to come to the **Tab A's Main screen**. So I used,

> Actions.popTo('A-Main');

### Actual behaviour

On performing this action, **undefined route is warning is returned and nothing happens**. Am i doing this in the right way?