vue-cli project compiles but "vue.version is undefined" error prevents opening in browser

I'm working on a project I set up with vue-cli (originally vue-cli-3 but I guess I just updated to vue-cli-4 while trying to fix this problem). It was working fine, but suddenly stopped opening in Firefox and Safari. "npm run serve" compiles successfully, but in the browser console it says that "vue.version is undefined". The problem seems to involve webpack, but I'm not sure (I haven't messed with those settings at all). I tried "npm update" but I'm not sure what else to try. It doesn't seem to be anything I did, since I removed new components back to what was working previously without affecting the problem, but I'm not sure what else it could be either.

Firefox error message:

Webpack 28
    __webpack_require__
    fn
    ./node_modules/cache-loader/dist/cjs.js?!./node_modules/babel-loader/lib/index.js!./node_modules/cache-loader/dist/cjs.js?!./node_modules/vue-loader/lib/index.js?!./src/components/LewisTester.vue?vue&type=script&lang=js&
    __webpack_require__
    fn
    ./src/components/LewisTester.vue?vue&type=script&lang=js&
    __webpack_require__
    fn
    vue
    __webpack_require__
    fn
    ./node_modules/cache-loader/dist/cjs.js?!./node_modules/babel-loader/lib/index.js!./node_modules/cache-loader/dist/cjs.js?!./node_modules/vue-loader/lib/index.js?!./src/App.vue?vue&type=script&lang=js&
    __webpack_require__
    fn
    ./src/App.vue?vue&type=script&lang=js&
    __webpack_require__
    fn
    vue
    __webpack_require__
    fn
    js
    __webpack_require__
    fn
    1
    __webpack_require__
    checkDeferredModules
    <anonymous>
    <anonymous>