NullPointerException thrown when inserting image bigger than attachment maximum size

Steps to reproduce:

1. Lower the Attachment Maximum Size (for testing purpose)
2. Add a new page/news
3. In the Rich Text/Wiki Editor, insert image
4. Select image with size bigger than Attachment Maximum Size
5. Click Attach

The following error message is caught:

java.lang.NullPointerException
 at com.atlassian.confluence.pages.actions.InsertImageInDraftAction.validateUpload(InsertImageInDraftAction.java:43)