Thanks for the sharing.
How I do debugging for my blocks normally is exactly what you mentioned - put in data reads at each step until the problem occur. In this case, the URL is not even registered in the API URL parameter block. And yes, for blocks with API and JSON or Object returns, I will use error or status blocks to check. The above example was just a minimally working example to replicate the error, not reflective of my practice.
Anyway, in the Thunkable Live update dated 10 Sep. the crash is resolved. The outstanding problem is the Live test on the desktop design page. But since the other testing modes are still working fine, so I let it be for the time being.