Fix 'error in invocation of downloads.show and downloads.open' in Chrome

This is a bug in webextension polyfill: mozilla/webextension-polyfill#138. It was fixed in v0.3.0

package.json

@@ -46,7 +46,7 @@
     "vue-router": "^3.0.1",
     "vuedraggable": "^2.18.1",
     "vuex": "^3.0.1",
-    "webextension-polyfill": "^0.2.1",
+    "webextension-polyfill": "^0.3.0",
     "xml2js": "^0.4.19"
   },
   "devDependencies": {

yarn.lock

@@ -10568,10 +10568,10 @@ wcwidth@^1.0.1:
   dependencies:
     defaults "^1.0.3"
 
-webextension-polyfill@^0.2.1:
-  version "0.2.1"
-  resolved "https://registry.yarnpkg.com/webextension-polyfill/-/webextension-polyfill-0.2.1.tgz#cdfc9126033039f1713553157d35beff1d4d6f4a"
-  integrity sha1-zfyRJgMwOfFxNVMVfTW+/x1Nb0o=
+webextension-polyfill@^0.3.0:
+  version "0.3.1"
+  resolved "https://registry.yarnpkg.com/webextension-polyfill/-/webextension-polyfill-0.3.1.tgz#fab2aed917a713a5d8221e41febad81c5d0b080f"
+  integrity sha512-ISB42vlgMyM7xE1u6pREeCqmmXjLsYu/nqAR8Dl/gIAnylb+KpRpvKbVkUYNFePhhXn0Obkkc3jasOII9ztUtg==
 
 webidl-conversions@^4.0.2:
   version "4.0.2"