Fix react warning

ui/app/app.js

@@ -214,6 +214,7 @@ App.prototype.renderNetworkDropdown = function () {
     h(
       DropdownMenuItem,
       {
+        key: 'main',
         closeMenu: () => this.setState({ isNetworkMenuOpen: !isOpen }),
         onClick: () => props.dispatch(actions.setProviderType('mainnet')),
       },
@@ -227,6 +228,7 @@ App.prototype.renderNetworkDropdown = function () {
     h(
       DropdownMenuItem,
       {
+        key: 'ropsten',
         closeMenu: () => this.setState({ isNetworkMenuOpen: !isOpen }),
         onClick: () => props.dispatch(actions.setProviderType('ropsten')),
       },
@@ -240,6 +242,7 @@ App.prototype.renderNetworkDropdown = function () {
     h(
       DropdownMenuItem,
       {
+        key: 'kovan',
         closeMenu: () => this.setState({ isNetworkMenuOpen: !isOpen }),
         onClick: () => props.dispatch(actions.setProviderType('kovan')),
       },
@@ -253,6 +256,7 @@ App.prototype.renderNetworkDropdown = function () {
     h(
       DropdownMenuItem,
       {
+        key: 'rinkeby',
         closeMenu: () => this.setState({ isNetworkMenuOpen: !isOpen }),
         onClick: () => props.dispatch(actions.setProviderType('rinkeby')),
       },
@@ -266,6 +270,7 @@ App.prototype.renderNetworkDropdown = function () {
     h(
       DropdownMenuItem,
       {
+        key: 'default',
         closeMenu: () => this.setState({ isNetworkMenuOpen: !isOpen }),
         onClick: () => props.dispatch(actions.setDefaultRpcTarget()),
       },
@@ -564,6 +569,7 @@ App.prototype.renderCommonRpc = function (rpcList, provider) {
       return h(
         DropdownMenuItem,
         {
+          key: `common${rpc}`,
           closeMenu: () => this.setState({ isNetworkMenuOpen: false }),
           onClick: () => props.dispatch(actions.setRpcTarget(rpc)),
         },