Java - no se abre pdf con netbeans

Hola saludos quisiera preguntar que puedo hacer para poder corregir el siguiente problema al correr el programa me sale lo siguiente y no me deja abrir el pdf estoy trabajando con netbeans,msql y itextpdf estoy creando un reporte pero el pdf no se abre y sale lo siguiente en el run de netbeans

run:
Error en PDFnull
jul 22, 2021 9:58:01 PM ventanas.RegistroAlumnos jButton6ActionPerformed
GRAVE: null
com.itextpdf.text.DocumentException: java.lang.NullPointerException
at com.itextpdf.text.pdf.PdfDocument.add(PdfDocument.java:821)
at com.itextpdf.text.Document.add(Document.java:277)
at ventanas.RegistroAlumnos.jButton6ActionPerformed(RegistroAlumnos.java:280)
at ventanas.RegistroAlumnos.access$400(RegistroAlumnos.java:36)
at ventanas.RegistroAlumnos$5.actionPerformed(RegistroAlumnos.java:113)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6533)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
at java.awt.Component.processEvent(Component.java:6298)
at java.awt.Container.processEvent(Container.java:2236)
at java.awt.Component.dispatchEventImpl(Component.java:4889)
at java.awt.Container.dispatchEventImpl(Container.java:2294)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
at java.awt.Container.dispatchEventImpl(Container.java:2280)
at java.awt.Window.dispatchEventImpl(Window.java:2746)
at java.awt.Component.dispatchEvent(Component.java:4711)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
Caused by: java.lang.NullPointerException
at com.itextpdf.text.pdf.PdfDocument.add(PdfDocument.java:438)
... 40 more

BUILD SUCCESSFUL (total time: 11 seconds)

que es lo que sucede y la palabra header me la marca en amarillo y en error que puedo hacer para arreglar este error