Uncaught Error: Element type is invalid: expected a string (for built-in components)

[kurtzilla]

I have just completed the 7th video of the series.
It is possible that I deleted the node_modules folder and re-ran npm install to make the following warning/error occur. I am on the hunt for the solution myself, but posting in the hopes that some else has had this issue and has been able to fix it.

I did try and rollback to a previous commit (hence the rm-rf node_modules) and haven't been able to fix the issue.

warning.js:36 Warning: React.createElement: type is invalid -- expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.
printWarning @ warning.js:36
warning @ warning.js:60
createElement @ ReactElementValidator.js:171
(anonymous) @ index.js:39
(anonymous) @ bundle.js:1363
__webpack_require__ @ bundle.js:556
fn @ bundle.js:87
(anonymous) @ multi_main:3
(anonymous) @ bundle.js:589
__webpack_require__ @ bundle.js:556
(anonymous) @ bundle.js:579
(anonymous) @ bundle.js:582
invariant.js:44 Uncaught Error: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in.
    at invariant (invariant.js:44)
    at ReactCompositeComponentWrapper.instantiateReactComponent [as _instantiateReactComponent] (instantiateReactComponent.js:77)
    at ReactCompositeComponentWrapper.performInitialMount (ReactCompositeComponent.js:367)
    at ReactCompositeComponentWrapper.mountComponent (ReactCompositeComponent.js:258)
    at Object.mountComponent (ReactReconciler.js:46)
    at mountComponentIntoNode (ReactMount.js:104)
    at ReactReconcileTransaction.perform (Transaction.js:140)
    at batchedMountComponentIntoNode (ReactMount.js:126)
    at ReactDefaultBatchingStrategyTransaction.perform (Transaction.js:140)
    at Object.batchedUpdates (ReactDefaultBatchingStrategy.js:62)

[kurtzilla]

Stranger still..
I just cloned a new local repo from the master, did npm install/npm start and am receiving the same error.

[ouss4m4]

i just started this tuto,
i get error in video numer 2 !!

just as i use Games (i didn't forget to import it )
everything crashed,

i'm thinking it's because the router is in alpha, something changed

[kelleysy]

This happened for me too... To fix it, I just made sure my react-router was the alpha
In package.json use:
"react-router": "4.0.0-alpha.5",

[kelleysy]

Also be sure to remove node_modules
rm -rf node_modules & then do npm install

[kurtzilla]

Thank you @kelleysy
I had tried changing the version as well, however, I had not removed the ^ (as in "^4.0.0-alpha.5")

==========================================
After a bit more sleuthing...

In this project, when I include '^4.0.0-alpha.6I receive the same error. Let's call this project, project A. I have another project using^4.0.0-alpha.6` as well, and it works just fine. Let's call it project B

After I do npm install, npm list react-router yields:
project A react-router@4.0.0-beta.5 (same when including ^4.0.0-alpha.5)
project B react-router@4.0.0-alpha.6

So something about this particular project, project A, is rewriting the version to beta
Both projects use react-scripts 0.8.5

Can anyone shed any light on why this project (project A) rewrites the version to beta.5?

[JohnnyKNL]

thanks @kelleysy this worked for me!