add plugin-proposal-optional-chaining

babel.config.js

@@ -1,3 +1,4 @@
 module.exports = {
   presets: ["@vue/cli-plugin-babel/preset"],
+  plugins: ["@babel/plugin-proposal-optional-chaining"],
 };

package.json

@@ -33,6 +33,7 @@
     "vuex-persistedstate": "^2.6.0"
   },
   "devDependencies": {
+    "@babel/plugin-proposal-optional-chaining": "^7.6.0",
     "@vue/cli-plugin-babel": "^4.0.5",
     "@vue/cli-plugin-eslint": "^4.0.5",
     "@vue/cli-plugin-pwa": "^4.0.5",

yarn.lock

@@ -339,6 +339,14 @@
     "@babel/helper-plugin-utils" "^7.0.0"
     "@babel/plugin-syntax-optional-catch-binding" "^7.2.0"
 
+"@babel/plugin-proposal-optional-chaining@^7.6.0":
+  version "7.6.0"
+  resolved "https://registry.yarnpkg.com/@babel/plugin-proposal-optional-chaining/-/plugin-proposal-optional-chaining-7.6.0.tgz#e9bf1f9b9ba10c77c033082da75f068389041af8"
+  integrity sha512-kj4gkZ6qUggkprRq3Uh5KP8XnE1MdIO0J7MhdDX8+rAbB6dJ2UrensGIS+0NPZAaaJ1Vr0PN6oLUgXMU1uMcSg==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.0.0"
+    "@babel/plugin-syntax-optional-chaining" "^7.2.0"
+
 "@babel/plugin-proposal-unicode-property-regex@^7.6.2":
   version "7.6.2"
   resolved "https://registry.yarnpkg.com/@babel/plugin-proposal-unicode-property-regex/-/plugin-proposal-unicode-property-regex-7.6.2.tgz#05413762894f41bfe42b9a5e80919bd575dcc802"
@@ -397,6 +405,13 @@
   dependencies:
     "@babel/helper-plugin-utils" "^7.0.0"
 
+"@babel/plugin-syntax-optional-chaining@^7.2.0":
+  version "7.2.0"
+  resolved "https://registry.yarnpkg.com/@babel/plugin-syntax-optional-chaining/-/plugin-syntax-optional-chaining-7.2.0.tgz#a59d6ae8c167e7608eaa443fda9fa8fa6bf21dff"
+  integrity sha512-HtGCtvp5Uq/jH/WNUPkK6b7rufnCPLLlDAFN7cmACoIjaOOiXxUt3SswU5loHqrhtqTsa/WoLQ1OQ1AGuZqaWA==
+  dependencies:
+    "@babel/helper-plugin-utils" "^7.0.0"
+
 "@babel/plugin-transform-arrow-functions@^7.2.0":
   version "7.2.0"
   resolved "https://registry.yarnpkg.com/@babel/plugin-transform-arrow-functions/-/plugin-transform-arrow-functions-7.2.0.tgz#9aeafbe4d6ffc6563bf8f8372091628f00779550"