Textarea not populating with previous value
After disabling WYSIWYG the text area started populating again. More digging revealed the reason it wasn't populating as a rich text box was because when an editor is used it submits the value as an array with two items; value and format. Consequently, this meant that the text editor was being given an array to display rather than a string.

Altering the #default_value to $sidebar_content[$key]['value'] resolved this issue.