deal with null Repository in getLastHistoryEntry()

fixes #3630

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/history/HistoryGuru.java

@@ -237,7 +237,10 @@ public History getHistoryUI(File file) throws HistoryException {
 
     public HistoryEntry getLastHistoryEntry(File file, boolean ui) throws HistoryException {
         final Repository repo = getRepository(file);
-        return repo.getLastHistoryEntry(file, ui);
+        if (repo != null) {
+            return repo.getLastHistoryEntry(file, ui);
+        }
+        return null;
     }
 
     /**

opengrok-indexer/src/test/java/org/opengrok/indexer/history/HistoryGuruTest.java

@@ -26,6 +26,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.opengrok.indexer.condition.RepositoryInstalled.Type.CVS;
 import static org.opengrok.indexer.condition.RepositoryInstalled.Type.MERCURIAL;
@@ -279,4 +280,11 @@ public void testScanningDepth() throws IOException {
 
         env.setScanningDepth(originalScanDepth);
     }
+
+    @Test
+    void testGetLastHistoryEntry() throws HistoryException {
+        HistoryGuru instance = HistoryGuru.getInstance();
+        File file = new File("/nonexistent");
+        assertNull(instance.getLastHistoryEntry(file, true));
+    }
 }