Fix layout editor of workflow nodes in Designer

It is currently not possible to customize the UI of workflow nodes in Designer, 404 errors are reported in the browser console.

How to reproduce:

• create a new workflow in Modeler named testworkflow and add a Accept / Reject node to it
• navigate to Designer to configure its Web UI layout under Layouts > Workflows > testworkflow in tab UI and click on the Accept/ Reject node layout to configure it

Expected result: the layout editor opens to configure the node layout.
Actual result: the layout editor does not open to configure the node layout and a 404 is reported in the browser console.