Updated the reducer (#431)

When the drawer opens / close, the result of AppNavigator.router.getStateForAction(action, state); is null. So the nav state in the store is null which causes errors. This solves that.

docs/guides/Redux-Integration.md

@@ -7,10 +7,13 @@ With redux, your app's state is defined by a reducer. Each navigation router eff
 ```
 const AppNavigator = StackNavigator(AppRouteConfigs);
 
+const navReducer = (state, action) => {
+  const newState = AppNavigator.router.getStateForAction(action, state);
+  return newState || state;
+};
+
 const appReducer = combineReducers({
-  nav: (state, action) => (
-    AppNavigator.router.getStateForAction(action, state)
-  ),
+  navReducer,
   ...
 });
 
@@ -55,4 +58,4 @@ const AppNavigator = StackNavigator({
 });
 ```
 
-In this case, once you `connect` `AppNavigator` to Redux as is done in `AppWithNavigationState`, `MyTabNavigator` will automatically have access to navigation state as a `navigation` prop.
+In this case, once you `connect` `AppNavigator` to Redux as is done in `AppWithNavigationState`, `MyTabNavigator` will automatically have access to navigation state as a `navigation` prop.