I discovered this right after buying B4 desktop (windows) and trying to load my last mockup. I am posting this just as a warning so someone doesnt lose work and get into this headache.
If you have a textfield, datagrid etc and have a DATE string as the fields, it will crash B4 with an exception as it tries to understand it.
Unhandled exception! System.InvalidCastException: Unable to cast object of type ‘System.DateTime’ to type ‘System.String’.
This will force-close the app, and even worse, it will continue crashing when you try to restart B4 and it reloads the project again.
You can also simply have a date string in the clipboard and try to paste it into B4 and it will crash.
Below is Sample data (DO NOT DO THIS UNLESS YOU WANT TO CRASH your desktop app):
Paste this anywhere to trigger a crash (text field, datagrid, even just the desktop):