$graph document failed to parse and gives misleading error

If I enter this URL in directly, it doesn't work

https://github.com/common-workflow-language/cwl-v1.2/blob/main/tests/revsort-packed.cwl

## Description

## Expected Behavior

It should display the workflow.

## Actual Behavior

It reports "Error: No workflow files were found in the given directory"

If I remove the file and give it the directory, then it works:

https://github.com/common-workflow-language/cwl-v1.2/blob/main/tests/

Other files in the same directory do work.

## Steps to Reproduce

1. Put https://github.com/common-workflow-language/cwl-v1.2/blob/main/tests/revsort-packed.cwl in the URL box