What version are you running?
3.0 dev
What's the URL of the page containing the problem?
/r/123/
What steps will reproduce the problem?

Open the Upload File dialog (either a new file using the Update -> Add File menu or by updating an existing file through Update File on a thumbnail)
Kill the dev server
Attempt to upload the file


What is the expected output? What do you see instead?
The code ungracefully fails with the following error:

uploadAttachmentView.js:72 Uncaught TypeError: Cannot read property 'err' of null



What operating system are you using? What browser?
Chrome (but any should trigger this)