No validation of a launch absence for running workflow with a custom tool

Scenario:

1. Open the "Application Settings" dialog in UGENE.
2. Add a custom tool that is a python script (see e.g. the attached *.xml-config and *.py-script).
3. Remove a path to python tool in the "Supported tools" section.
4. Create a workflow with the added custom tool and run it.
   Current result: the workflow is launched and is finished with an error status, but without an error message. There is a message in the log like this:

   Running external process: "" "/Users/username/Desktop/my_tool.py" "/var/folders/0l/nvxqy6vn35s9y87800qgsr6c0000gp/T/ugene_tmp/p31254/wd_external/tmpin1568794567.txt" "/var/folders/0l/nvxqy6vn35s9y87800qgsr6c0000gp/T/ugene_tmp/p31254/wd_external/tmpout1568794567.tmp"
   

   Expected result: the workflow didn't pass the validation.