Minor: fix a DemoBench crash that can occur if the web server fails to start.

tools/demobench/src/main/kotlin/net/corda/demobench/views/NodeTerminalView.kt

@@ -166,20 +166,18 @@ class NodeTerminalView : Fragment() {
             webServer.open(config).then {
                 Platform.runLater {
                     launchWebButton.graphic = null
-                }
-                it.match({
+                    it.match(success = {
                         log.info("Web server for ${config.legalName} started on $it")
-                    Platform.runLater {
                         webURL = it
                         launchWebButton.text = "Reopen\nweb site"
                         app.hostServices.showDocument(it.toString())
-                    }
-                }, {
+                    }, failure = {
                         launchWebButton.text = oldLabel
                     })
                 }
             }
         }
+    }
 
     private fun launchRPC(config: NodeConfig) = NodeRPC(
         config = config,