Just FYI: I tried this on my iPad 3 and iPhone 5. I went into the Apple Notes app and selected to define a word. I was prompted with the blank screen in the dictionary popup. I downloaded the Oxford American English Dictionary without issues. I returned to the text, reselected to define the word, and everything worked perfectly. Then I tried selecting a word to define from within Marvin and it also worked perfectly. I don't know what all this means, but obviously the bug doesn't occur for everyone as I had no issues on either of my iOS 7 devices.