title: NullPointerException in itextpdf during Form flattening with Acrofields
body: <p>I have a six page PDF with some acrofields on each page.
  I have deleted page 4 and 5 using following code :</p>
  
  <pre><code>List&lt;Integer&gt; pagesToKeep = Arrays.asList(new int[]{ 1, 2, 3 });
  pdfReader.selectPages(pagesToKeep);
  </code></pre>
  
  <p>But then on Form freezing using following command :</p>
  
  <pre><code> pdfStamper.setFreeTextFlattening(true);
       pdfStamper.close();
  </code></pre>
  
  <p>I am getting NullPointerException. 
  Reason for this :  On Form Flattening , Acro Fields defined on pages are unable to locate the page number, as Page 6 does not exists any more.</p>
  
  <p>Exception Trace : </p>
  
  <pre><code>java.lang.NullPointerException
      at com.itextpdf.text.pdf.PdfStamperImp.flatFields(PdfStamperImp.java:1019)
      at com.itextpdf.text.pdf.PdfStamperImp.close(PdfStamperImp.java:227)
      at com.itextpdf.text.pdf.PdfStamper.close(PdfStamper.java:231)
      at com.ally.bankapi.service.impl.DocumentServiceImpl.generateDocument(DocumentServiceImpl.java:1272)
      at com.ally.bankapi.service.impl.DocumentServiceImplTest.testGenerateDocumentSuccess(DocumentServiceImplTest.java:111)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:606)
      at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
      at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
      at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
      at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
      at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
      at org.junit.runners.BlockJUnit4ClassRunner.runNotIgnored(BlockJUnit4ClassRunner.java:79)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:71)
      at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
      at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
      at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
      at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
      at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
      at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
      at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
      at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
      at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
      at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
      at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
      at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
  </code></pre>