Proper tab in SystemBrowser gets destroyed when trying to change to it with unsaved changes

[linusha]

When opening two tabs in a System Browser so that it is possible to switch nicely between them, and then making some changes in one of the editors, trying to switch tabs again (without clicking save!) and then, when being warned that there are unsaved changes, clicking cancel instead of throwing the changes away, the second tab is deleted.

Expected behavior: The switch between the tabs does not occur, but all tabs remain.