If the document contains lists with applied customs style, when trying to open the Manage Styles Dialog using the button in the ribbon, ArgumentNullException is thrown and handled internally, and the dialog is not opened. If the user clicks the button multiple times (the number of lists with custom styles), the dialog is opened.

Steps to reproduce:
1. Create a custom list style.

2. Add some test text in the RadRichTextBox.

3. Apply the custom list style in two places in the document.

4. Click on the Change Styles button.

Observed: Dialog is not opened.

Expected: The dialog should open.