This bug is occurring on Google Chrome Version 52.0.2743.116 (64-bit) on macOS Sierra when the browser window is maximized (i.e. it is its own desktop). Does not occur on Safari.
After generating Umple source code and using the resulting about:blank page to surf the web, Umple source code cannot be regenerated until that tab is closed.
(Sorry if this is too specific of a bug.)
Umple source is generated in a separate new tab.
The browser focuses on tab that the Umple code was previously generated in, and no code is generated.