Hi Felipe,
Greetings from Syncfusion.
We have checked your code snippet and found that you have called the setState() inside the future builder and it is updating the whole chart again and the properties of the chart will get reset each time. That is why it is throw a null check exception. We have modified your code snippet like storing the data by parsing the JSON at the initState and assigned it to the data source now the trackball working fine. We have shared the sample below for your reference.
Screenshot:
Regards,
Yuvaraj.
