Wrong error message when system/type is unknown, but "start dump" is clicked

[gingerbeardman]

Using latest AppVeyor build.

• System == unknown
• Type == unknown

Actual

• If I scan for disks, start dumping button becomes enabled
• If I press it, I get the error "a complete dump already exists"
• But there was no existing dump

Expected

• I would expect to get the error "system and disc type are not set"
• Maybe "start dump" should not be enabled in this situation?

Screen

[mnadareski]

Made sure that the selected item is not NONE before re-enabling the button.