Script Registry throws error when workflow function implements CannedScript

package com.onresolve.scriptrunner.canned.jira.workflow.postfunctions

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue
import com.onresolve.scriptrunner.canned.CannedScript
import com.onresolve.scriptrunner.canned.jira.utils.ConditionUtils
import com.onresolve.scriptrunner.canned.util.BuiltinScriptErrors
import com.onresolve.scriptrunner.canned.util.SimpleBuiltinScriptErrors
import com.opensymphony.util.TextUtils
import org.apache.log4j.Logger

class Sample implements CannedScript {

    Logger log = Logger.getLogger(Sample)
    public static final String FIELD_SOME_CONFIG = 'FIELD_SOME_CONFIG'

    String getName() {
        "Sample"
    }

    String getDescription() {
        "Log stuff"
    }

    List getCategories() {
        ["Function"]
    }


    List getParameters(Map params) {
        [
                ConditionUtils.getConditionParameter(),
                [
                        Name       : FIELD_SOME_CONFIG,
                        Label      : 'Text to log',
                        Description: 'This will be written to the debug log',
                        Type       : 'text'
                ]
        ]

    }

    public BuiltinScriptErrors doValidate(Map params, boolean forPreview) {
        BuiltinScriptErrors errorCollection = new SimpleBuiltinScriptErrors()
        if (!params[FIELD_SOME_CONFIG]) {
            errorCollection.addError(FIELD_SOME_CONFIG, "You must provide debug text.")
        }
        errorCollection
    }

    Map doScript(Map params) {
        MutableIssue issue = params['issue'] as MutableIssue
        String text = params[FIELD_SOME_CONFIG] as String

        Boolean doIt = ConditionUtils.processCondition(params[ConditionUtils.FIELD_CONDITION] as String, issue, false, params)
        if (!doIt) {
            return [:]
        }

        log.debug(text)

        [:]
    }

    String getDescription(Map params, boolean forPreview) {
        String htmlResult = ''
        if (params[ConditionUtils.FIELD_CONDITION] as String != '') {
            htmlResult += 'If condition: <pre>' + TextUtils.htmlEncode(params[ConditionUtils.FIELD_CONDITION] as String) + '</pre> is true:<br>'
        }
        htmlResult += getName() + " log the text ${params[FIELD_SOME_CONFIG]}"
        return htmlResult
    }

    public Boolean isFinalParamsPage(Map params) {
        true
    }
}