Instructions:

• Enable the SSO plugin in stash-config.properties:

  # Whether SSO support should be enabled or not. Regardless of this setting SSO authentication 
  # will only be activated when a Crowd directory is configured in Stash that is configured 
  # for SSO.
  plugin.auth-crowd.sso.enabled=true
  

• Add the Crowd directory to Stash.
• Any further navigation will result in a NPE.

Moving forward:

• Disabled the plugin parameter.
• Restarted Stash without issues.

Finally:

• Entered a value for SSO Domain.
• Enabled the plugin.
• Restarted Stash successfully.
• Entered a SSO domain in Crowd.