title: ComboBoxModel - removeItem method for fireIntervalRemoved throws exception
body: <p>I've created a ComboBoxModel class which extends AbstractListModel. I can add item to the combobox, but when I try to remove, I get an exception</p>
  
  <blockquote>
    <p>Exception in thread "AWT-EventQueue-0"
    java.lang.IllegalArgumentException: null source</p>
  </blockquote>
  
  <p>at line </p>
  
  <blockquote>
    <p>this.fireIntervalRemoved(selectedItem, itemIndex, itemIndex);</p>
  </blockquote>
  
  <pre><code>public class TComboBoxModel extends AbstractListModel implements ComboBoxModel {
  
      private int itemIndex; 
  
      private Object selectedItem = null;
      private ArrayList&lt;Object&gt; itemList;
  
      public TComboBoxModel() {
          itemList = new ArrayList&lt;&gt;();
      }
  
      public void addItem(String item) {
          this.itemList.add(item);
          this.fireIntervalAdded(item, itemIndex, itemIndex);
      }
  
      public void removeItem() {
          if (itemIndex &gt;= 0 &amp;&amp; itemIndex &lt; getSize()) {
              this.itemList.remove(itemIndex);
              this.fireIntervalRemoved(selectedItem, itemIndex, itemIndex);
          }
      }
  
      @Override
      public void setSelectedItem(Object anObject) {
          if ((selectedItem != null &amp;&amp; !selectedItem.equals(anObject)) || selectedItem == null &amp;&amp; anObject != null) {
              this.selectedItem = anObject;
              this.fireContentsChanged(anObject, -1, -1);
          }
      }
  
      @Override
      public Object getSelectedItem() {
          return selectedItem;
      }
  
      @Override
      public int getSize() {
          return itemList.size();
      }
  
      @Override
      public Object getElementAt(int index) {
          return itemList.get(index).toString();
      }
  
      public int getItemIndex() {
          return itemIndex;
      }
  
      public void increaseItemIndex() {
          itemIndex++;
      }
  
      public void decreaseItemIndex() {
          itemIndex--;
      }
  
  }
  </code></pre>