Marble crash when cancelling layer generation

[activityworkshop]

When running marble, if you select a layer for the first time (eg satellite view) then a progress bar pops up saying that it may take some time to generate the layer. If you press the cancel button, then marble crashes with a "Floating Point Exception".

If the system is running well then this dialog disappears again quickly so you have to be quick to reproduce this error. But if the disk takes time to spin up or it's a bit slow for some other reason then it's tempting to press cancel.

[hamish]

seems fixed in the latest version.