There is a small but nasty bug with importing files for use as design maps or heightfields.
I went to import a RAW file for use as a design map. I didn't remember to change the file size - leaving it at zero. So, naturally, I got an error and and error message. No problem there.
However, the error message would not go away. You cannot acknowledge the message and go back and fix the error - it stays on the screen. In fact, the only way to get out of the situation is to call up the Windows Task Manager and kill the process.
I suspect there may be other ways to get to that error message, but I suspect that getting out of it will be the same in all cases - call up the Task Manager to kill the job.