You could make the UI hidden. 

Since it's a Generator of id, you don't have to manually assign the value.

Thanks for the help. It works now. Actually I still need to assign the final value of the  ID Generator field to another field in the form for display only. But seems like I can only do this after the form is submitted. So what I did was assign it later on in the workflow. And everything works now. Thanks again.

Hi Herman Susilo
To hide the Auto Id field, click "Hidden?" property field.
To display the generated number later on in another form (and form is tied to the same db table), you can use a textfield (set to readonly) or a customHTML like "#form.mytable.runningNumberId#" to display.

The id generator plugin does these things:
1) create the field in db,
2) take the value from env variable, add 1 and store into db,
3) update env variable with the latest counter value.

Subsequent to this, you can use textfield or textarea to display this field value in your form. Just remember to set it as readonly to prevent user from changing the value.