After previewing the form, close the document system will pop up a "save or not" prompt

In SAP Business ByDesign System, I developed a custom business object and developed the preview form functionality on it.

Now there is a problem that when I click the Edit button to enter a instance, I do not modify any fields, then click the preview button to preview the form, and then click the close button to close the instance, the system will pop up a "save or not" prompt.

However, when I tested the standard business object, I found that the standard business object did not have this problem. The standard business object would directly close the instance without the prompt box of whether to save or not.