open in editor fix, show tip instead of error

Summary:
if REACT_EDITOR not set we need to show a pro tip. For now error is shown

now:

<img width="967" alt="screen shot 2017-04-11 at 14 48 06" src="https://cloud.githubusercontent.com/assets/1488195/24905942/da7ae876-1ec6-11e7-8f4d-a2505ee1958f.png">

after fix:

<img width="641" alt="screen shot 2017-04-11 at 14 49 38" src="https://cloud.githubusercontent.com/assets/1488195/24905957/e60637d6-1ec6-11e7-92ad-ba03dbf3ebff.png">

1) REACT_EDITOR is not set
2) use unknown or terminal editor (I use neovim)
3) make a red box in UIExplorer
4) click on stacktrace line
Closes react/react-native#13443

Differential Revision: D4963762

Pulled By: hramos

fbshipit-source-id: 8425d6e7c6f786e8ac51ebc9b7d721bb69dd7313

Author: skv-headless

server/util/launchEditor.js

@@ -109,7 +109,7 @@ function guessEditor() {
     return [process.env.EDITOR];
   }
 
-  return null;
+  return [null];
 }
 
 function printInstructions(title) {