RxRouting.ResolveView should not use exception handling as flow control.

Currently, `RxRouting.ResolveView` attempts to find various possible types for the view given a type for the view model, and throws an exception each time one is not found. However, this is not really an exceptional situation, as different types for the view are possible by decision. 

I have changed it so that it simply test for `null` instead of throwing.