Hi mates,
I also had the same problem some hours ago until I realized I was doing something wrong so the app didn't initialize properly.
Make sure you create a folder with de app name, let's say <Test>, then your python script must have the same name that the folder in which is contained, in this case, <Test.py> (is case sensitive so make sure to respect uppercases)
Below I copy the most basic piece of code which works perfectly for me (I just tested it before writing this)