Avoid possible NPE if TagTreePointer points at direct structure element

Author: yulian-gaponenko

kernel/src/main/java/com/itextpdf/kernel/pdf/tagutils/TagTreePointer.java

@@ -721,7 +721,8 @@ PdfStructElem getCurrentStructElem() {
             throw new PdfException(PdfException.TagTreePointerIsInInvalidStateItPointsAtFlushedElementUseMoveToRoot);
         }
 
-        if (currentStructElem.getPdfObject().getIndirectReference().isFree()) { // is removed
+        PdfIndirectReference indRef = currentStructElem.getPdfObject().getIndirectReference();
+        if (indRef != null && indRef.isFree()) { // is removed
             throw new PdfException(PdfException.TagTreePointerIsInInvalidStateItPointsAtRemovedElementUseMoveToRoot);
         }