ID 483619 - Previewing a Filter with a Mask containing custom Nodes can cause a Render Error

Problem summary
Previewing a Filter with a Mask containing custom Nodes can cause a Render Error

Customer reported version
Mari 4.7v3

Customer reported platform
Windows 10

Steps to reproduce

1) Extract the contents of the attached 'dummy_function.zip' file to the Mari 'Scripts' folder

2) Launch Mari and open any project

3) Create a dummy_function_node and a Channel node

4) Connect the dummy_function_node's output to the Channel node's input

5) Create a Paint node and apply any Filter to it

6) In the Filter dialog, add the Channel as a Mask and enable Preview

Expected behaviour
A preview of the Filter's result should be rendered in the Canvas

Actual behaviour
Red-screen 'Unable to Render / Shader Error'

Workaround
Unknown.

Reproduced by support
This bug has been reproduced in:
4.7v4 - Windows 10 - CentOS 7.7
4.7v1 - Windows 10
4.6v1 - Windows 10
4.5v1 - Windows 10
4.2v1 - Windows 10
4.0v1 - Windows 10 - CentOS 7.7

Earliest version tested
4.0v1 - This issue appears to be in all versions of the product