To reproduce:
- Create a new winforms project.
- Drop a RadRichTextEditor on the form.
- Drop a button on the form.
- In the button Click event add: radRichTextEditor1.ShowSpellCheckingDialog();
- Run the project.
- Click the button - a message is displayed "The spelling check is complete".
- Type some text with a spelling error in the rich text editor.
- Click the button again - a System.InvalidOperationException exception occurs.

Workaround:
radRichTextEditor1.RichTextBoxElement.SpellCheckingDialog = new SpellCheckingDialog();