Using a non existing workflow variable in a node makes it impossible to deploy/download a project

Steps to reproduce:

• Have a workflow
• Create a workflow variable? For example, toto
• Add a start node, and end node and an Approve node
• In the Approve node, go to the "form" tab, and drag drop this toto variable
• Save the workflow
• Click the Download button => all is good

Now:

• Delete the variable
• Save the workflow
• Click the Download button => you get this error (see attachement) and fill your whole Studio project is broken for ever

Restore your workflow:

• Open the "Approve" node
• Remove the widget from the Form
• Save
• Download => all is good again

Notice: Before "downloading", I could not, of course, hot reload my server with the project, hence, wanted to give a try to the manual installation to explore what was wrong

(was hesitating between "bug" and "improvement". because, after all, it's me doing stupid thing such as deleting a wf variable used in a node. But I found this error a bit scary, isn't it? )