There's currently an issue when choosing the Material UI theme, which causes two versions of react to be loaded on the page. There is a fix coming soon in NPM, but I won't know for sure until it lands.
Workaround
For now, run npm dedupe after the scaffolder finishes running, and restart the dev server (if you previously started it), which should fix the issue in the meantime.
Background / Cause
There's currently an issue when choosing the Material UI theme, which causes two versions of react to be loaded on the page. There is a fix coming soon in NPM, but I won't know for sure until it lands.
Workaround
For now, run
npm dedupe
after the scaffolder finishes running, and restart the dev server (if you previously started it), which should fix the issue in the meantime.