General cleanup of handling for WidgetMessaging instances

When setting, delete the old one. Because delete checks if it exists first, we can safely call this.

The change in FromWidgetPostMessageApi is just something noticed while debugging.

src/stores/ActiveWidgetStore.js

@@ -116,6 +116,8 @@ class ActiveWidgetStore extends EventEmitter {
     }
 
     setWidgetMessaging(widgetId, wm) {
+        // Stop any existing widget messaging first
+        this.delWidgetMessaging(widgetId);
         this._widgetMessagingByWidgetId[widgetId] = wm;
         this.emit('update');
     }