Tabs button in Paragraph Properties dialog is visible even when RadRichTextBox.TabStopsPropertiesDialog is not set. In addition, pressing the button in this scenario closes the Paragraph Properties dialog itself.

As this property is automatically set using MEF, this situation could happen if the tabs dialog is missing in the the composition container.

Workaround: Check whether the TabStopsPropertiesDialog is null and if true, then hide the TabsButton:

    var windowTabStopsPropertiesDialog = (this.radRichTextBox.TabStopsPropertiesDialog as RadWindow);
    var windowParagraphPropertiesDialog = (this.radRichTextBox.ParagraphPropertiesDialog as RadWindow);
    if (windowTabStopsPropertiesDialog == null)
    {
        (windowParagraphPropertiesDialog.FindName("TabsButton") as RadButton).Visibility = Visibility.Collapsed;
    }