SXSSFSheet: beware of null _writer

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1879551 13f79535-47bb-0310-9956-ffa450edef68

build.gradle

@@ -127,6 +127,7 @@ subprojects {
         exclude '**/BaseTestCellUtil.class'
         exclude '**/TestUnfixedBugs.class'
         exclude '**/TestOneFile.class'
+        include '**/TestDeferredSXSSFWorkbook.class'
 
         // Exclude Test Suites
         exclude '**/All*Tests.class'

src/ooxml/java/org/apache/poi/xssf/streaming/SXSSFSheet.java

@@ -1880,7 +1880,7 @@ public class SXSSFSheet implements Sheet
             SXSSFRow row = _rows.get(firstRowNum);
             // Update the best fit column widths for auto-sizing just before the rows are flushed
             _autoSizeColumnTracker.updateColumnWidths(row);
-            _writer.writeRow(rowIndex, row);
+            if (_writer != null) _writer.writeRow(rowIndex, row);
             _rows.remove(firstRowNum);
             lastFlushedRowNumber = rowIndex;
         }