Fix for listener bug

[devotaaabel]

One fix to consider for preventing inability to add listeners to dispatcher before it's initialized is initializing it earlier. Whether this fix is adopted or not, I think we need to add more information to the documentation about when the user can safely add listeners and start navigation.

cc. @danesfeder @ericrwolfe

[danesfeder]

@devotaaabel I think initializing the dispatcher earlier is a good idea as it doesn't require any parameters.

Because we changed the way we add the listeners (packed in NavigationViewOptions), a crash as a result from trying to add a listener to a null dispatcher should be prevented.

But it would probably be a good idea to add some documentation or a runtime exception to ensure NavigationView#startNavigation(NavigationViewOptions options) isn't called until the onNavigationReadyCallback.onNavigationReady() callback fires.

[devotaaabel]

@danesfeder Those are probably both good ideas (adding documentation and a runtime exception).