Don't trigger Web3 Provider change when second Provider Endpoint choice dialog was canceled

src/execution-context.js

@@ -114,16 +114,15 @@ function ExecutionContext () {
   this.executionContextChange = function (context, endPointUrl, cb) {
     if (!cb) cb = () => {}
     function runPrompt () {
-      executionContext = context
       if (!endPointUrl) {
         endPointUrl = 'http://localhost:8545'
       }
       modalDialogCustom.prompt(null, 'Web3 Provider Endpoint', endPointUrl, (target) => {
+        executionContext = context
         setProviderFromEndpoint(target)
         self.event.trigger('contextChanged', ['web3'])
         cb()
       }, () => {
-        self.event.trigger('contextChanged', ['web3'])
         cb()
       })
     }