How to turned a multi-branch Jenkins Job into a Job DSL (Conversion: Trouble with Multi-Branch Job)

Question

Jenkins Job DSL Conversion: Trouble with Multi-Branch Job

I'm currently working on converting a multi-branch Jenkins job into a Job DSL. Here's what I've written so far:

multibranchPipelineJob('example') {
    branchSources {
        git {
            id('123456789') // IMPORTANT: use a constant and unique identifier
            remote('https://github.com/jenkinsci/job-dsl-plugin.git')
            credentialsId('github-ci')
            includes('JENKINS-*')
        }
    }
    orphanedItemStrategy {
        discardOldItems {
            numToKeep(20)
        }
    }
}

However, I'm encountering an error upon execution:

ERROR: (unknown source) No signature of method: org.jenkinsci.plugins.workflow.multibranch.WorkflowMultiBranchProject.getProperty() is applicable for argument types: (java.lang.String) values: [nectar.plugins.rbac.groups.JobProxyGroupContainer]
Possible solutions: hasProperty(java.lang.String), getProperties(), getProperties(), addProperty(com.cloudbees.hudson.plugins.folder.AbstractFolderProperty)

The expected output is a Job DSL that can be used to create and build a multi-branch job. Could anyone point me in the right direction to resolve this error? Any guidance would be greatly appreciated.