Since from version 3.1 Export Layout take forever (30 to 60 min) to parse Preparing export in place of few minutes., make the ability to stop the process when seeing a problem after clicking the Export button.
For now, all the process of Preparing export have to go before the Stop button became available. With versions before 3,1 it wasn't a problem since the phase preparing export doesn't take forever but not, please activate the stop at the beginning of the process.