Fix #425 janeway freezing

I believe the problem was that we had an document listener on the field that was checking the fields as we were adding to it, which would cause a weird collision type thing. 
What I basically did was put a lock around the code of setting the estimate field and changing requirements, which would limit the usage of check fields while switching requirements. Seems to work, but should have thorough testing to make sure this bug is now being avoided.

Fixes #425