This is a known issue that we will have a fix for, hopefully by the end of the week. Hopefully.
In the interim, you have two options.
1) Upgrade to Redux 4.2 via our GitHub repository (the version that will be released after some more internal testing) here: https://github.com/reduxframework/redux-framework
2) Install and activate a plugin from the wp.org repository called “Classic Widgets,” which will revert the widgets screen to the old version. That is a stop-gap suggestion until we can get this new release finished up and the issue solved.
If you go with the first option and discover something wrong, please use the GitHub issue tracker to report it until the final release.
Thanks!