Fix bug in grep method

common/src/main/java/ctbrec/StringUtil.java

@@ -83,6 +83,9 @@ public class StringUtil {
                 result.append(line).append('\n');
             }
         }
-        return result.deleteCharAt(result.length() - 1).toString();
+        if (result.length() >= 2) {
+            result = result.deleteCharAt(result.length() - 1);
+        }
+        return result.toString();
     }
 }