DevTools should not crawl unmounted subtrees when profiling starts (facebook#23162)

Previously we crawled all subtrees, even not-yet-mounted ones, to initialize context values. This was not only unecessary, but it also caused an error to be thrown. This commit adds a test and fixes that behavior.

Author: Brian Vaughn

packages/react-devtools-shared/src/__tests__/profilerStore-test.js

@@ -221,4 +221,24 @@ describe('ProfilerStore', () => {
     expect(data.commitData).toHaveLength(1);
     expect(data.operations).toHaveLength(1);
   });
+
+  it('should not throw while initializing context values for Fibers within a not-yet-mounted subtree', () => {
+    const promise = new Promise(resolve => {});
+    const SuspendingView = () => {
+      throw promise;
+    };
+
+    const App = () => {
+      return (
+        <React.Suspense fallback="Fallback">
+          <SuspendingView />
+        </React.Suspense>
+      );
+    };
+
+    const container = document.createElement('div');
+
+    utils.act(() => legacyRender(<App />, container));
+    utils.act(() => store.profilerStore.startProfiling());
+  });
 });

packages/react-devtools-shared/src/backend/renderer.js

@@ -1347,11 +1347,19 @@ export function attach(
   // Fibers only store the current context value,
   // so we need to track them separately in order to determine changed keys.
   function crawlToInitializeContextsMap(fiber: Fiber) {
-    updateContextsForFiber(fiber);
-    let current = fiber.child;
-    while (current !== null) {
-      crawlToInitializeContextsMap(current);
-      current = current.sibling;
+    const id = getFiberIDUnsafe(fiber);
+
+    // Not all Fibers in the subtree have mounted yet.
+    // For example, Offscreen (hidden) or Suspense (suspended) subtrees won't yet be tracked.
+    // We can safely skip these subtrees.
+    if (id !== null) {
+      updateContextsForFiber(fiber);
+
+      let current = fiber.child;
+      while (current !== null) {
+        crawlToInitializeContextsMap(current);
+        current = current.sibling;
+      }
     }
   }