It appears that the example for multi-monitor (https://docs.inductiveautomation.com/display/DOC80/Multi-Monitor+Clients) has some mistakes in the final example comments.

The comments in the final example describes the returned value of system.gui.getScreens() as being both a dataset and a tuple. However, the returned value is actually a list, per https://docs.inductiveautomation.com/display/DOC79/system.gui.getScreens and the script console.