[Lazarus] Modal Window Crashes on Mac OS X 10.15 Beta (Catalina)

[tobiasgiesen at gmail.com]

Hello,

I have been very happy with how my app can be compiled for 64-bit Cocoa
so far, but on Mac OS Catalina Beta it crashes. Even the most minimal
application with only two forms crashes if you try to open the second
form as a modal dialog. It happens with both Lazarus 2.0.2 and trunk.

The error description in the crash report is quite detailed but I have
no idea what needs to be done:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Cannot update for observer <NSFrontmostDocumentWindowObserver 0x7fff8ec52208> for the key path "mainWindow.representedURL" from <TCocoaApplication 0x100d089a0>, most likely because the value for the key "mainWindow" has changed without an appropriate KVO notification being sent. Check the KVO-compliance of the TCocoaApplication class.'
Performing @selector(actionButtonClick:) from sender TCocoaButton 0x100b127e0
abort() called
terminating with uncaught exception of type NSException

It happens inside the call to beginModalSessionForWindow in
TCocoaWidgetSet.StartModal.

What can be done?

Cheers,
Tobias