SpellChecker relies on Silverlights's IsolatedStorage when adding words to the dictionary on user's demand. If it is disabled, throws an Exception.
In case you want to use the "Add Word" functionality of the spell checker with disabled IsolatedStorage, you'll have to use a non-persistent (session only) storage of the custom words like so:
radRichTextBox.SpellChecker = new DocumentSpellChecker(new RadNonPersistentCustomDictionary());
Another option would be to disable the spell checking altogether by setting the IsSpellCheckingEnabled to false. Do not hesitate to contact us again in case you have any other issues with the use of RadRichTextBox.
the Telerik team
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