Do not allow adding non token addresses

ui/app/add-token.js

@@ -142,7 +142,13 @@ AddTokenScreen.prototype.render = function () {
               if (!valid) return
 
               const { address, symbol, decimals } = this.state
+              this.checkIfToken(address.trim())
+              .then(() => {
                 this.props.dispatch(actions.addToken(address.trim(), symbol.trim(), decimals))
+              })
+              .catch((reason) => {
+                this.setState({ warning: 'Not a valid token address.' })
+              })
             },
           }, 'Add'),
         ]),
@@ -202,6 +208,12 @@ AddTokenScreen.prototype.validateInputs = function () {
   return isValid
 }
 
+AddTokenScreen.prototype.checkIfToken = async function (address) {
+  const contract = this.TokenContract.at(address)
+  const result = await contract.balance(address)
+  return result[0].toString()
+}
+
 AddTokenScreen.prototype.attemptToAutoFillTokenParams = async function (address) {
   const contract = this.TokenContract.at(address)