Add a move issue into my current workflow, and when trigger during transition, system show a error message of "MoveIssueFunction fails due to ...: root cause: java.lang.NullPointerException"

Have make sure:
1. Both source and destination project uses the same screen scheme
2. Permission to create issue is set to everyone in both project
3. System use only 1 set of issue types for all project
4. Both source and destination project have the status

I want to know at that line where the plugin throw the error, what actually is NULL?

The back end catalina log look like :

Root cause:
java.lang.NullPointerException
at com.minyaa.workflow.function.MoveIssueFunction.execute(MoveIssueFunction.java:40)
at com.opensymphony.workflow.AbstractWorkflow.executeFunction(AbstractWorkflow.java:1050)
at com.opensymphony.workflow.AbstractWorkflow.transitionWorkflow(AbstractWorkflow.java:1446)
at com.opensymphony.workflow.AbstractWorkflow.doAction(AbstractWorkflow.java:564)
at com.atlassian.jira.workflow.OSWorkflowManager.doWorkflowActionInsideTxn(OSWorkflowManager.java:905)
at com.atlassian.jira.workflow.OSWorkflowManager.doWorkflowAction(OSWorkflowManager.java:865)
at com.atlassian.jira.bc.issue.DefaultIssueService.transition(DefaultIssueService.java:449)
at com.atlassian.jira.web.action.issue.CommentAssignIssue.doExecute(CommentAssignIssue.java:196)
at webwork.action.ActionSupport.execute(ActionSupport.java:165)
at com.atlassian.jira.action.JiraActionSupport.execute(JiraActionSupport.java:82)
at webwork.interceptor.DefaultInterceptorChain.proceed(DefaultInterceptorChain.java:39)
at webwork.interceptor.NestedInterceptorChain.proceed(NestedInterceptorChain.java:31)
at webwork.interceptor.ChainedInterceptor.intercept(ChainedInterceptor.java:16)
...