RadSpell's dialog should have proper tabIndex order to provide keyboard navigation. You can find attached a sample that lets you provide the desired order. When the buttons are disabled the browser should not focus them (i.e., the disabled attribute should prevent this even if tabIndexes are not modified). Related to this improvement request: http://feedback.telerik.com/Project/108/Feedback/Details/126935
The error (Uncaught TypeError: Cannot read property 'mouseup' of undefined or SCRIPT5007: Unable to get value of the property 'mouseup': object is null or undefined ) manifests in Chrome and IE9 only and only if you have a debugger on, otherwise the page simply disposes without issues.
You can find more information about the problem, cause and repro in the following thread: http://www.telerik.com/community/forums/spell-check-hander-error-the-length-of-the-string-exceeds-the-value-set-on-the-maxjsonlength-property
The issue is preventing the users from adding a word to RadSpell custom dictionary. A workaround is attached. You can apply the DialogScriptFile property to all RadSpell instances through an ASP Theme. The bug will be fixed in Q1 2015 SP1, so as soon as you upgrade to this release (or a later one), you should remove the workaround.