Without knowing anything about your environment it's hard to tell, but there are two standouts in your post:
> After ejecting the ISO
Do you mean the ISO that is the disk image of your Ubuntu system? You ejected this and wonder why the VM crashed? That's like unplugging the hard drive on any system, expecting it to keep running.
The second is that the error message you're getting is the standard OS message when an app crashes. What do do with it? click one of the options and move on - OK will just acknowledge the crash; Reopen will attempt to relaunch the app. Not sure what more I can add to this.