Backport child navigation cache fix

src/getChildNavigation.js

@@ -1,6 +1,7 @@
 import getChildEventSubscriber from './getChildEventSubscriber';
 import getChildRouter from './getChildRouter';
 import getNavigationActionCreators from './routers/getNavigationActionCreators';
+import getChildrenNavigationCache from './getChildrenNavigationCache';
 import invariant from './utils/invariant';
 
 const createParamGetter = route => (paramName, defaultValue) => {
@@ -14,8 +15,7 @@ const createParamGetter = route => (paramName, defaultValue) => {
 };
 
 function getChildNavigation(navigation, childKey, getCurrentParentNavigation) {
-  const children =
-    navigation._childrenNavigation || (navigation._childrenNavigation = {});
+  const children = getChildrenNavigationCache(navigation);
 
   const childRoute = navigation.state.routes.find(r => r.key === childKey);
 

src/getChildrenNavigationCache.js

@@ -0,0 +1,16 @@
+export default function getChildrenNavigationCache(navigation) {
+  if (!navigation) {
+    return {};
+  }
+
+  let childrenNavigationCache =
+    navigation._childrenNavigation || (navigation._childrenNavigation = {});
+  let childKeys = navigation.state.routes.map(route => route.key);
+  Object.keys(childrenNavigationCache).forEach(cacheKey => {
+    if (!childKeys.includes(cacheKey)) {
+      delete childrenNavigationCache[cacheKey];
+    }
+  });
+
+  return navigation._childrenNavigation;
+}

src/getNavigation.js

@@ -1,5 +1,6 @@
 import getNavigationActionCreators from './routers/getNavigationActionCreators';
 import getChildNavigation from './getChildNavigation';
+import getChildrenNavigationCache from './getChildrenNavigationCache';
 
 export default function getNavigation(
   router,
@@ -38,6 +39,7 @@ export default function getNavigation(
       };
     },
     dangerouslyGetParent: () => null,
+    _childrenNavigation: getChildrenNavigationCache(getCurrentNavigation()),
   };
 
   const actionCreators = {