ID 370538 - Mask Items disappears when layers are added/deleted with "Only display the shaders that are associated to the selected items" enabled

Problem summary

Mask Items disappears when layers are added or deleted with the "Only display the shaders that are associated to the selected items" enabled. 

Customer reported version

Modo 12.1v2

Customer reported platform
Windows 10 

Steps to reproduce

1) Create a sphere.

2) Right-Click the sphere in the Items list. Select Shade->Create Item Mask. 

3) Go to the Shading tab. Add any layer and then remove the layer. 

4) The Item Mask should now have disappeared from the Shader Tree. 

5) Refresh the Shader Tree and the Item Mask should now be visible again. 

Workaround
Disable "Only display the shaders that are associated to the selected items" in the Shading tab. 

Reproduced by support

This bug has been reproduced in:

Modo 12.1v2 - Windows 10 - CentOS 7 - MacOSX 11.13 
Modo 12.0v1 - Windows 10
Modo 11.2v3 - Windows 10
Modo 11.1v1 - Windows 10
Modo 11.0v3 - Windows 10
Modo 10.2v4 - Windows 10 - CentOS 7 - MacOSX 11.13  

Earliest version tested

Modo 10.2v4 - This issue seems to appear in all versions of Modo. 

Expected behaviour
The Mask Item should not disappear from the Shader Tree if layers are added to it or removed from it. 

Actual behaviour
Layers added or removed from Mask Item makes it disappear from the Shader Tree.