Add assertions

src/main/java/pony/Pony.java

@@ -13,6 +13,7 @@ public class Pony {
      * @param filePath File path of the memory file.
      */
     public Pony(String filePath) {
+        assert filePath != null : "File path should not be empty";
         ui = new Ui();
         storage = new Storage(filePath);
         tasks = new TaskList(storage.loadTaskList());

src/main/java/pony/TaskList.java

@@ -32,6 +32,7 @@ public Task getTask(int index) throws PonyException {
         if (index < 0 || index >= sizeOf()) {
             throw new PonyException("Task index invalid!!");
         }
+        assert index < sizeOf() && index >= 0 : "Task index should be valid";
         return tasks.get(index);
     }