Steps to reproduce:

1. class WidgetTemperature extends CWidget {
       onInitialize() {
           this._button_callback_valid = false;
           var temperatureButton = document.getElementById('temperatureButton');
           if (temperatureButton) {
               if (!this._button_callback_valid) {
                   this._button_callback_valid = true;
                   temperatureButton.addEventListener('click', function()
   Unknown macro: {                    var temperatureTextbox = document.getElementById('temperatureTextbox');                    alert(temperatureTextbox.value);                    // sending temperature to WebSocket server                    const socket = new WebSocket('ws}

   );
               }
           }
   */        
       }
       
       onClearContents()

   {     }

   }

Result:
Button hanlder is not called if adding code for button handling in onInitialize(). But button hanlder is called if adding code for button handling in setContents().
Expected:
Button hanlder is called if adding code for button handling in onInitialize().