The credential plugin config is not initialized properly.

I found that the code:
"resolveCredentials();"
need to be executed in the code in order for the credentials to load properly.
I do not have enough knowledge abt your plugin to know exactly where to insert it, but I added the above line to the getCredentials method in the GraniteClientConfig.java file and it works fine for now.

Error Stacktrace:

Caused by: java.lang.AssertionError: Credentials were not resolved
at org.jenkinsci.plugins.graniteclient.GraniteClientConfig.getCredentials(GraniteClientConfig.java:146)
at org.jenkinsci.plugins.graniteclient.GraniteClientExecutor.execute(GraniteClientExecutor.java:87)
at org.jenkinsci.plugins.graniteclient.GraniteClientExecutor.execute(GraniteClientExecutor.java:69)
at org.jenkinsci.plugins.graniteclient.PackageChoiceParameterDefinition.getPackageList(PackageChoiceParameterDefinition.java:134)
... 164 more