Fix or silence "cannot find fiber with ID" warnings when hovering

[gaearon]

Sometimes I see "cannot find fiber with ID" when hovering over the tree. They are sporadic. I'll need to dig into why they happen. But we also need to look at our usage of warnings to make sure they're either actionable or gated by DEV/DEBUG.

[bvaughn]

I agree this is probably not ideal currently. What are "expected" edge cases and what are legit errors? I've tried to replace legit error cases with throw statements recently, but I understand that it's still not the clearest.

Some (most?) of the the remaining warnings are expected potential edge cases. A little more context...

For example, if an element is selected in the DevTools frontend, then it gets unmounted on the page/backend, we may temporarily see this warning when we query for its inspected props:

react-devtools-experimental/src/backend/renderer.js

Lines 1432 to 1438 in 20193f0

	function inspectElementRaw(id: number): InspectedElement | null {
	let fiber = idToFiberMap.get(id);
	if (fiber == null) {
	console.warn(`Could not find Fiber with id "${id}"`);
	return null;
	}

Similarly, since the Store is mutable, it's possible that a node will be removed while a render batch is in progress:

react-devtools-experimental/src/devtools/views/Components/Element.js

Lines 122 to 128 in 85179fe

	// Handle elements that are removed from the tree while an async render is in progress.
	if (element == null) {
	console.warn(`<ElementView> Could not find element at index ${index}`);
	// This return needs to happen after hooks, since hooks can't be conditional.
	return null;
	}

[bvaughn]

This repository is being merged into the main React repo (github.com/facebook/react). As part of this, I am moving all issues to that repository as well and closing them here.

This issue has been relocated to:
facebook/react#16493