ethereum-boilerplate: ERROR: no-unsafe-optional-chaining

I followed the setup instructions exactly but I get the error:

ERROR in src/components/Contract/Contract.jsx
  Line 90:34:  Unsafe usage of optional chaining. If it short-circuits with 'undefined' the evaluation will throw TypeError  no-unsafe-optional-chaining

I also get 99 warnings about URLs that look like:

WARNING in ./node_modules/antd/dist/antd.css (./node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[1].oneOf[5].use[1]!./node_modules/postcss-loader/dist/cjs.js??ruleSet[1].rules[1].oneOf[5].use[2]!./node_modules/source-map-loader/dist/cjs.js!./node_modules/antd/dist/antd.css)
Module Warning (from ./node_modules/source-map-loader/dist/cjs.js):
Failed to parse source map: 'webpack://antd/./components/time-picker/style/index.less' URL is not supported
 @ ./node_modules/antd/dist/antd.css 8:6-231 22:17-24 26:7-21 58:25-39 59:36-47 59:50-64 63:6-73:7 64:54-65 64:68-82 70:42-53 70:56-70 72:21-28 83:0-201 83:0-201 84:22-29 84:33-47 84:50-64 61:4-74:5
 @ ./src/App.jsx 18:0-28
 @ ./src/index.js 6:0-24 24:35-38

I have uninstalled and reinstalled yarn, and deleted and redid the ethereum-boilerplate setup. No matter what, I get 99 warnings and this error. I am on Manjaro Linux.