Resolve fiber === null in certain app structures / conditions

- Resolves an issue in the event processing pipeline where fibers return null and result in a critical error (loading fails, event queue continues to grow)
- Applies a band-aid solution to a "knock-on" effect of the fix that results in missing / invalid fibers in active trees / fiber maps, causing the UI render to fail in REMPL

Author: Som Meaden

src/data/process-events.ts

@@ -201,6 +201,13 @@ export function processEvents(
       }
 
       case "update":
+        // This check is potentially required (??) due to an unsupported app
+        // structure (e.g. multiple roots) or otherwise invalid react code.
+        // Knock-on effects of this check are mitigated in `../ui/fiber-maps`
+        if (!fiberById.has(event.fiberId)) {
+          continue;
+        }
+
         updateCount++;
 
         fiber = fiberById.get(event.fiberId) as MessageFiber;

src/ui/utils/fiber-maps.tsx

@@ -75,7 +75,20 @@ export const useFiberChildren = (
 ) => {
   const { selectTree } = useFiberMaps();
   const tree = selectTree(groupByParent, includeUnmounted);
-  const leaf = tree.getOrCreate(fiberId);
+  let leaf = tree.getOrCreate(fiberId);
+
+  // Requesting the root node (fiber 0) returns an invalid fiber?
+  // This state is potentially (??) introduced due to an unsupported app
+  // structure and a subsequent workaround in `../data/process-events`
+  if (!fiberId && leaf?.fiber == null) {
+    // find the next node in the tree with a valid fiber and children
+    for (const [activeFiberId, { fiber, firstChild }] of tree.nodes) {
+      if (fiber && firstChild) {
+        leaf = tree.getOrCreate(activeFiberId);
+        break;
+      }
+    }
+  }
 
   const compute = React.useCallback(() => leaf.children || EMPTY_ARRAY, [leaf]);