generate heatmap and nade trajectories - invalid memory address or nil pointer dereference

[AXNTROYUANXD]

I was using the test scripts in the heatmap example and the nade trajectory example.

Got this error:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x8295c2]

goroutine 1 [running]:
main.main()
        .../playground/test_go_scripts/nade_trajectories.go:128 +0x562
exit status 2

The demo I used is here.