PDFBOX-6009: avoid NPE; improve javadoc

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1925702 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

pdfbox/src/main/java/org/apache/pdfbox/multipdf/Splitter.java

@@ -235,7 +235,11 @@ private void cloneStructureTree(PDDocument destinationDocument) throws IOExcepti
         PDNumberTreeNode dstNumberTreeNode = new PDNumberTreeNode(PDParentTreeValue.class);
         dstNumberTreeNode.setNumbers(dstNumberTreeAsMap);
         dstStructureTreeRoot.setParentTree(dstNumberTreeNode);
-        dstStructureTreeRoot.setParentTreeNextKey(dstNumberTreeNode.getUpperLimit() + 1);
+        Integer upperLimit = dstNumberTreeNode.getUpperLimit();
+        if (upperLimit != null)
+        {
+            dstStructureTreeRoot.setParentTreeNextKey(upperLimit + 1);
+        }
         dstStructureTreeRoot.setClassMap(srcStructureTreeRoot.getClassMap());
         cloneRoleMap(srcStructureTreeRoot, dstStructureTreeRoot);
         cloneIDTree(srcStructureTreeRoot, dstStructureTreeRoot);

pdfbox/src/main/java/org/apache/pdfbox/pdmodel/common/PDNumberTreeNode.java

@@ -276,7 +276,7 @@ public void setNumbers( Map<Integer, ? extends COSObjectable> numbers )
     /**
      * Get the highest value for a key in the number map.
      *
-     * @return The highest value for a key in the map.
+     * @return The highest value for a key in the map or null if missing.
      */
     public Integer getUpperLimit()
     {
@@ -317,7 +317,7 @@ private void setUpperLimit( Integer upper )
     /**
      * Get the lowest value for a key in the number map.
      *
-     * @return The lowest value for a key in the map.
+     * @return The lowest value for a key in the map or null if missing.
      */
     public Integer getLowerLimit()
     {