I have the same problem.  I looked at the log file, and X-plane thinks I have filed a flight plan. I did not.  The error message seems to appear because I'm not following the " flight plan". I looked at the Log file and found the following line: p1(N01XP) Popup FP filed to KSDF at 35000 via 6107N11751W YHY YSM YKJ YVC YQD LEATS YDN VLR HML IDJ ST
What is a popup FP (file plan?).  How do I remopve it?
Thanks