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 (capturedErrors !== null && capturedErrors.size > 0) {
      while (nextUnitOfWork !== null) {
        if (hasCapturedError(nextUnitOfWork)) {
          
          nextUnitOfWork = performFailedUnitOfWork(nextUnitOfWork);
        } else {
          nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
        }
        if (nextUnitOfWork === null) {
          invariant(
            pendingCommit !== null,
            'Should have a pending commit. This error is likely caused by ' +
              'a bug in React. Please file an issue.',
          );
          
          priorityContext = TaskPriority;
          commitAllWork(pendingCommit);
          priorityContext = nextPriorityLevel;
          if (capturedErrors === null || capturedErrors.size === 0) {
            
            
            
            break;
          }