To reproduce:
1. Create a slave node with a label name LABEL.
2. Create a matrix job.
3. In the matrix job, add a slave in the Configuration Matrix section.
4. Choose a label or node from the dropdowns in this section.

You will see something like this in each reference to the slave node and label:

nodename (null)

The string (null) used to be the description from the slave node configuration. Now it is the string 'null'. This is shown in the attachment.