Handle flow errors

Author: jackpope

packages/react-native-renderer/src/ReactFiberConfigFabric.js

@@ -647,8 +647,12 @@ FragmentInstance.prototype.observeUsing = function (
   traverseFragmentInstance(this._fragmentFiber, observeChild, observer);
 };
 function observeChild(instance: Instance, observer: IntersectionObserver) {
+  const publicInstance = getPublicInstance(instance);
+  if (publicInstance == null) {
+    throw new Error('Expected to find a host node. This is a bug in React.');
+  }
   // $FlowFixMe[incompatible-call] Element types are behind a flag in RN
-  observer.observe(getPublicInstance(instance));
+  observer.observe(publicInstance);
   return false;
 }
 // $FlowFixMe[prop-missing]
@@ -669,8 +673,12 @@ FragmentInstance.prototype.unobserveUsing = function (
   }
 };
 function unobserveChild(instance: Instance, observer: IntersectionObserver) {
+  const publicInstance = getPublicInstance(instance);
+  if (publicInstance == null) {
+    throw new Error('Expected to find a host node. This is a bug in React.');
+  }
   // $FlowFixMe[incompatible-call] Element types are behind a flag in RN
-  observer.unobserve(getPublicInstance(instance));
+  observer.unobserve(publicInstance);
   return false;
 }
 
@@ -693,8 +701,7 @@ export function commitNewChildToFragmentInstance(
 ): void {
   if (fragmentInstance._observers !== null) {
     fragmentInstance._observers.forEach(observer => {
-      // $FlowFixMe[incompatible-call] Element types are behind a flag in RN
-      observer.observe(getPublicInstance(child));
+      observeChild(child, observer);
     });
   }
 }

packages/react-reconciler/src/ReactFiberTreeReflection.js

@@ -345,19 +345,19 @@ export function doesFiberContain(
   return false;
 }
 
-export function traverseFragmentInstance<A, B, C>(
+export function traverseFragmentInstance<I, A, B, C>(
   fragmentFiber: Fiber,
-  fn: (Instance, A, B, C) => boolean,
+  fn: (I, A, B, C) => boolean,
   a: A,
   b: B,
   c: C,
 ): void {
   traverseFragmentInstanceChildren(fragmentFiber.child, fn, a, b, c);
 }
 
-function traverseFragmentInstanceChildren<A, B, C>(
+function traverseFragmentInstanceChildren<I, A, B, C>(
   child: Fiber | null,
-  fn: (Instance, A, B, C) => boolean,
+  fn: (I, A, B, C) => boolean,
   a: A,
   b: B,
   c: C,

scripts/error-codes/codes.json

@@ -543,5 +543,6 @@
   "555": "Cannot requestFormReset() inside a startGestureTransition. There should be no side-effects associated with starting a Gesture until its Action is invoked. Move side-effects to the Action instead.",
   "556": "Expected prepareToHydrateHostActivityInstance() to never be called. This error is likely caused by a bug in React. Please file an issue.",
   "557": "Expected to have a hydrated activity instance. This error is likely caused by a bug in React. Please file an issue.",
-  "558": "Client rendering an Activity suspended it again. This is a bug in React."
+  "558": "Client rendering an Activity suspended it again. This is a bug in React.",
+  "559": "Expected to find a host node. This is a bug in React."
 }