Fix bug when the array of SortFields contains a null SortField on it

bobo-browse/src/main/java/com/browseengine/bobo/sort/SortCollector.java

@@ -212,19 +212,27 @@ private static SortField convert(Browsable browser, SortField sort) {
     }
   }
 
+  protected static SortField buildSortFieldFromQuery(Query q) {
+      if (q != null && !(q instanceof MatchAllDocsQuery)) {
+        return SortField.FIELD_SCORE;
+      } else {
+        return SortField.FIELD_DOC;
+      }
+  }
+
   public static SortCollector buildSortCollector(Browsable browser, Query q, SortField[] sort,
       int offset, int count, boolean fetchStoredFields, Set<String> termVectorsToFetch,
       String[] groupBy, int maxPerGroup, boolean collectDocIdCache) {
     if (sort == null || sort.length == 0) {
-      if (q != null && !(q instanceof MatchAllDocsQuery)) {
-        sort = new SortField[] { SortField.FIELD_SCORE };
-      } else {
-        sort = new SortField[] { SortField.FIELD_DOC };
-      }
+      sort = new SortField[] { buildSortFieldFromQuery(q) };
     }
 
     boolean doScoring = false;
     for (SortField sf : sort) {
+      if (sf == null) {
+          sf = buildSortFieldFromQuery(q);
+      }
+
       if (sf.getType() == SortField.Type.SCORE) {
         doScoring = true;
         break;

bobo-browse/src/test/java/com/browseengine/bobo/sort/SortCollectorTest.java

@@ -0,0 +1,19 @@
+package com.browseengine.bobo.sort;
+
+import junit.framework.TestCase;
+
+import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.MatchAllDocsQuery;
+import org.apache.lucene.search.SortField;
+
+/**
+ * @author mdelapenya
+ */
+public class SortCollectorTest extends TestCase {
+
+    public void testBuildSortFieldFromQuery() {
+        assertEquals(SortCollector.buildSortFieldFromQuery(null), SortField.FIELD_DOC);
+        assertEquals(SortCollector.buildSortFieldFromQuery(new MatchAllDocsQuery()), SortField.FIELD_DOC);
+        assertEquals(SortCollector.buildSortFieldFromQuery(new BooleanQuery()), SortField.FIELD_SCORE);
+    }
+}