Item remove or event handler set to null may cause mouse events to cease working.

[ppieczul]

item.remove() or item.onMouse..... = null does not check if item is included in the project and always decrements the item event handler counters (_itemEvents).
When item is part of a group, and that group is removed, and later this item is removed, the counters will go below zero.
When counters go below zero, on subsequent correct registration of the event handlers, the counters will be incremented, but eventually, they will reach zero. If that happens, events will stop working.

The workaround is to always check if item isInserted() below calling remove or setting handlers to null.
I think however, than these functions should be safe to call even if item is not in the project, and should never cause internal incoherency that later has unexpected consequences.