Call super constructor

src/backends/widget.js

@@ -18,6 +18,8 @@ import WidgetApi from 'matrix-widget-api';
 
 class WidgetBackend extends BaseBackend {
   constructor(widgetId, userId) {
+    super();
+
     this.userId = userId;
 
     this.api = new WidgetApi(widgetId);