I see there is an event handler on the Ticket entity. Please share that microflow too.
And on other hand, use microflow debugging to find out at what step the error actually occurs so that its easier to isolate the cause.
https://docs.mendix.com/howto6/debug-microflows
You have a NullPointer exception. Please check if your object exists before changing it in a microflow.
Add a split at the start of the flow, and check if $Ticket != empty