Three points from discussion in https://github.com/reactjs/react-redux/issues/756:

* Due to https://bugs.chromium.org/p/chromium/issues/detail?id=701371, environments like CodeSandbox show `Error: null` in our “React caught an error” warning. Let’s consider skipping both as they’re not very useful and look like a bug. `Error: null` looks like a separate error and draws attention away from the real error just above.

* In any case we should probably not print `The error was thrown at:` if we don’t have a JS stack.

* There is a related issue. What do we pass to error boundary if we caught `null`? It seems that most error boundaries will intuitively do something like `if (this.state.error)` or read `error.message`, which `null` will break. There are other falsy values too like `''` and `undefined`. My proposal is we always wrap primitives into our own `Error` objects that we pass to the boundary.
if (
        node.pendingWorkPriority === NoWork ||
        node.pendingWorkPriority > priorityLevel
      ) {
         
        shouldContinue = true;
        node.pendingWorkPriority = priorityLevel;
      }
      if (node.alternate !== null) {
        if (
          node.alternate.pendingWorkPriority === NoWork ||
          node.alternate.pendingWorkPriority > priorityLevel
        ) {
           
          shouldContinue = true;
          node.alternate.pendingWorkPriority = priorityLevel;
        }
      }
      if (node.return === null) {
        if (node.tag === HostRoot) {
          const root: FiberRoot = (node.stateNode: any);
          scheduleRoot(root, priorityLevel);
          if (!isPerformingWork) {
            switch (priorityLevel) {
              case SynchronousPriority:
                 
                if (isUnbatchingUpdates) {