Fix the assertion that makes the test case fail.

src/main/java/harmony/mastermind/logic/commands/DeleteCommand.java

@@ -51,7 +51,12 @@ public CommandResult execute() {
             model.clearRedoHistory();
 
         } catch (TaskNotFoundException | IndexOutOfBoundsException tnfe) {
-            assert false : "The target task cannot be missing";
+            // by A0138862W: 
+            // uncommented this line because it makes the DeleteCommandTest fail at line 33
+            // should return invalid message for UI to display otherwise it'll display null
+            //assert false : "The target task cannot be missing";
+
+            return new CommandResult(Messages.MESSAGE_INVALID_TASK_DISPLAYED_INDEX);
         }
 
         return new CommandResult(String.format(MESSAGE_DELETE_TASK_SUCCESS, toDelete));

src/test/java/guitests/DeleteCommandTest.java

@@ -27,9 +27,9 @@ public void delete() {
         currentList = TestUtil.removeTaskFromList(currentList, targetIndex);
         targetIndex = currentList.length/2;
         assertDeleteSuccess(targetIndex, currentList);
-
+        
         //invalid index
-        commandBox.runCommand("delete " + currentList.length + 1);
+        commandBox.runCommand("delete " + (currentList.length + 1));
         assertResultMessage("The task index provided is invalid");
 
     }