Removing an item with Connection can cause Connection Form to misbehave

User can open Connection Form, keep it open and then eventually remove all items with Connections, the Form will stay up and will allow user to assign target and signal to a non-existant Connection node, causing an error ("Uknown property "target"") to pop up. 

After attempting to change signal: 


The Form will stay after the error occurs, will keep displaying signals, targets and "To" selection will be interactable.