avoid duplicate Webchannels, bring back default WebChannel, opt-out via window.CHANNEL=null

Author: hhaensel

assets/js/channels.js

@@ -36,6 +36,12 @@ Genie.WebChannels.sendMessageTo = async (channel, message, payload = {}) => {
 Genie.initWebChannel = function(channel = Genie.Settings.webchannels_default_route) {
   // A message maps to a channel route so that channel + message = /action/controller
   // The payload is the data exposed in the Channel Controller
+
+  if (Genie.AllWebChannels.map(x=>x.channel).includes(channel)) {
+    console.warn("Channel '" + channel + "' already exists. Please use another name.");
+    return
+  }
+  
   var WebChannel = {};
   WebChannel.sendMessageTo = async (channel, message, payload = {}) => {
     let msg = JSON.stringify({
@@ -406,7 +412,10 @@ function isDev() {
 }
 
 // --------------- Initialize WebChannel ---------------
-// compatibilty with Stipple v0.28
-if (window.CHANNEL !== undefined) {
+// compatibilty with earlier Genie versions and with Stipple v0.28
+if (window.CHANNEL === undefined) {
+  Genie.initWebChannel(Genie.Settings.webchannels_default_route || '____');
+} else if (typeof window.CHANNEL == 'string') {
   Genie.initWebChannel(window.CHANNEL);
 }
+// if window.CHANNEL == null, the user is responsible for initializing the WebChannel