NullPointerException on Pie Chart gadget using scripted issue picker cf

Summary
A NullPointerException error was showed on Pie Chart gadget when a scripted issue picker custom field is selected on the Statistic Type

Steps to reproduce the problem

1. Create a new custom field of issue picker from script runner
2. Create a Pie Chart gadget on the dashboard 
3. Select a JQL filter and scripted issue picker custom field in Statistic Type
4. Press on Save
5. It shows on the gadget a big red error (Please see PieChartError.log)

Expected Behaviour
The Pie Chart gadget should be worked fine and display something as per the screenshot below:

Actual Behaviour
A NullPointerException error was showed