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.
}
    
    
    
    return null;
  }
  function performUnitOfWork(workInProgress: Fiber): Fiber | null {
    
    
    
    
    const current = workInProgress.alternate;
    
    if (__DEV__) {
      startWorkTimer(workInProgress);
    }
    let next = beginWork(current, workInProgress, nextPriorityLevel);
    if (__DEV__ && ReactFiberInstrumentation.debugTool) {
      ReactFiberInstrumentation.debugTool.onBeginWork(workInProgress);
    }
    if (next === null) {
      
      next = completeUnitOfWork(workInProgress);
    }
    ReactCurrentOwner.current = null;
    if (__DEV__) {
      ReactDebugCurrentFiber.resetCurrentFiber();
    }
    return next;
  }