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It seems churlish to just close the bug and blame the problem on ckeditor - even though it is their problem.
By the simple replacement of a couple of lines of code in "./themes/default/widgets/rich_textarea/rich_textarea.php" we can error trap ckeditor's bug and feed a sensible result back to P4A.
Of course, as you say in the bug, P4A can't know if a theme exists or not - but it doesn't need to. My small change will feed back a sensible result to P4A and stop it crashing.
If we want P4A to work properly and help the developer build applications quickly, surely we should do all we can to make life as easy as possible?
I assume that because rich_textarea.php is part of ckeditor, you don't want to interfere with it to ease future upgrades? You still need to edit line 89 in any case ("filebrowserBrowseUrl:").
rich_textarea.php is not a part of ckeditor but that's not the point, report the bug to the right place, which is ckeditor