This behaviour is the same as "Snap!" from which Cellular is based upon. Is this not the behaviour that we want? Do you want to instead show the Costume edit screen?
What error are you talking about? Do you mean "This XML file does not appear to have any style information associated..."? This should occur, and isn't an error. It just says that its displaying the XML without styling, which is what we want.
Cellular HTML5: a few of the on-state sprite menu choices do not work.
"Duplicate" works OK.
"Delete" works OK.
"Edit" does not seem to do anything.
"Export..." produces the error show at the bottom image.