Type fix

Author: Ashoat

flow-typed/npm/react-navigation_v2.x.x.js

@@ -1175,6 +1175,6 @@ declare module 'react-navigation' {
     dispatch: NavigationDispatch,
     actionSubscribers: Set<NavigationEventCallback>,
     getScreenProps: () => {},
-    getCurrentNavigation: () => NavigationScreenProp<State>,
+    getCurrentNavigation: () => ?NavigationScreenProp<State>,
   ): NavigationScreenProp<State>;
 }

src/middleware.js

@@ -124,7 +124,7 @@ function createNavigationPropConstructor(key: string) {
     dispatch: NavigationDispatch,
     state: State,
     router: NavigationRouter<*, *>,
-    getCurrentNavigation: () => NavigationScreenProp<State>,
+    getCurrentNavigation: () => ?NavigationScreenProp<State>,
   ): NavigationScreenProp<State> => {
     invariant(
       router,