Fixes #2881: Chat button is no longer optional in no jQuery version. Thanks to @ma-jahn ❤️

public/assets/chat/chat-no-jquery.coffee

@@ -467,7 +467,9 @@ do(window) ->
         @renderBase()
 
       # disable open button
-      document.querySelector(".#{ @options.buttonClass }").classList.add @inactiveClass
+      btn = document.querySelector(".#{ @options.buttonClass }")
+      if btn
+        btn.classList.add @inactiveClass
 
       @setAgentOnlineState 'online'
 
@@ -770,7 +772,10 @@ do(window) ->
     onError: (message) =>
       @log.debug message
       @addStatus(message)
-      document.querySelector(".#{ @options.buttonClass }").classList.add('zammad-chat-is-hidden')
+      btn = document.querySelector(".#{ @options.buttonClass }")
+      if btn
+        btn.classList.add('zammad-chat-is-hidden')
+
       if @isOpen
         @disableInput()
         @destroy(remove: false)

public/assets/chat/chat-no-jquery.js

@@ -739,10 +739,14 @@ var extend = function(child, parent) { for (var key in parent) { if (hasProp.cal
     };
 
     ZammadChat.prototype.render = function() {
+      var btn;
       if (!this.el || !document.querySelector('.zammad-chat')) {
         this.renderBase();
       }
-      document.querySelector("." + this.options.buttonClass).classList.add(this.inactiveClass);
+      btn = document.querySelector("." + this.options.buttonClass);
+      if (btn) {
+        btn.classList.add(this.inactiveClass);
+      }
       this.setAgentOnlineState('online');
       this.log.debug('widget rendered');
       this.startTimeoutObservers();
@@ -1088,9 +1092,13 @@ var extend = function(child, parent) { for (var key in parent) { if (hasProp.cal
     };
 
     ZammadChat.prototype.onError = function(message) {
+      var btn;
       this.log.debug(message);
       this.addStatus(message);
-      document.querySelector("." + this.options.buttonClass).classList.add('zammad-chat-is-hidden');
+      btn = document.querySelector("." + this.options.buttonClass);
+      if (btn) {
+        btn.classList.add('zammad-chat-is-hidden');
+      }
       if (this.isOpen) {
         this.disableInput();
         return this.destroy({