ID 414276 - Mari crashes when using 'Channel Mask' with a channel not connected to a shader input

Problem summary
Mari crashes when using 'Channel Mask' with a channel which is not connected to a shader input

Customer reported version
mari.4.6v1

Customer reported platform
windows10

Steps to reproduce

1) Open Mari and create a new project 

2) Add a channel and make sure it is not connected to a shader input 

3) Go to the Painting Palette, turn on the 'Channel Mask' and under Mask Channel select the channel created in step 2

result: Instant crash

Expected behaviour
Mari should not crash and it should use the channel regardless if whether it is connected to a shader input

Actual behaviour
Mari crashes 

Workaround
Connect the channel to a shader input 

Reproduced by support
This bug has been reproduced in:

4.6v1 - Windows 10 - CentOS 7 -regression 

Unable to reproduce bug in:
4.5v2 - Windows 10 - CentOS 7

Earliest version tested
4.5v2 - This issue doesn't appear in this version and has regressed