Error while creating records from plugin.

Dear All,

We have a 1:N relationship between Policy and CustomNote entity.

-->  We have another plugin which triggers on Associate Message, which creates the records in this CustomNote Entity.

-->  We have created a plugin which will triggers on CREATE Message of 'CustomNote' entity to autogenarate the sequence no. for created record (ID of target entity is necessary as we are fetching fields from target entity).

Now when i am creating records through Associate plugin, this sequence generator plugin is showing error of 'Object reference is not setting to instance'.

can anybody help me with following question.

1. is it necessary to provide policyid(targetentityid) while creating records from plugin (1:N between policy&Customnotes)?

2. If yes, then its a lookup. How can i define it?

3. is there any other way to handle this issue?

Thanks in advance