There is a warning in the tutorial rendered by HarryH, that once you open the editor you should not close it without importing a mission file or it will cause an error to WOFF. Not sure if this is still the case because his comments were related to WOFF3 and not WOFF UE. Hopefully someone will update the tutorial for WOFF_UE
If you have the latest version of Mission Editor, i.e. WOFF UE Mission editor v1.0, it is not true anymore. You can open mission file or not without any problem. The pdf tutorial by HarryH, especially WOFF error part was for WOFF3 Mission editor only. I am affraid, my English is very poor, so I am not able to create/update Mission Editor manual and I was glad that HarryH was so helpful and created one.