How to reproduce:

• Create a new multi-configuration project
• Add a user-defined axis:
  • Name: axis1
  • Value: value1 value2
• Add a combination filter:
  • axis1 != 'value2'
• Save the configuration
• "values2" is not displayed in status page.

This is caused by null MatrixConfiguration is passed via built-in hudson/project/matrix tag, as MatrixProject#getItem returns null for inactive axes combination.
This often causes NPE for plugins using hudson/project/matrix.