'Cancel' doesn't work when uploading patterns to EightByEight

[cibomahto]

Steps to reproduce:

1. Connect eightbyeight
2. Run patternpaint, from 'welcome' screen, select eightbyeight, be sure to load example patterns
3. Click the upload button
4. On the 'Blinky Exporter' dialog, click cancel
5. Dialog momentarily disappears, then reappaers

Expected behavior: Upload is cancelled (or dialog pops up to ask for conformation)?