I have also been seeing this randomly will all my Xojo apps since installing macOS 10.14.
To me this seems like a macOS bug since I never had this problem before 10.14.
Sometimes it takes closing a particular window 3 or 4 times but eventually it crashes with:
System Integrity Protection: enabled
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Application Specific Information:
Application Name(1238,0x1126565c0) malloc: Incorrect checksum for freed object 0x7fdf13e5e370: probably modified after being freed.
Corrupt value: 0x7