The binding editor canvas does not clear upon deleting the last remaining binding

[jdaignault]

Upon deleting the only remaining binding from the binding editor, the canvas will still display nodes from the binding that was just deleted. Additionally, compiling the blueprint when it's in this state may lead to the editor displaying errors and/or crashing.

Steps to reproduce:

1. Create a new widget blueprint
2. Add a new binding containing a singular Call a Function destination node in the binding canvas, and assign its function to anything
3. Delete the newly created binding
4. Observe that the canvas still contains the Call a Function destination node, despite no bindings remaining
5. Compile the blueprint
6. Observe that occasionally the editor will exhibit errors/crash behavior