Reports of possible bugs should be posted to Report an Issue where they will be seen by FileMaker tech personnel.
Are you adding a field previously defined in the database to your layout or defining a brand new field?
Making a design change by defining a brand new field is not a safe practice to engage in while others are using the database (or while a script is modifying data...) Though it shouldn't trigger a crash.
Thank you for your post.
The posting from PhilModJunk is correct. You should not modify the structure of a database file while it is being actively shared. With that said, please post the header of the crash report plus the Thread that crashed. This may provide a reason why FileMaker Pro crashed.