The element directly under the cursor at the time of the right-click, or null when
the click landed on empty space or on an element that is not exposed through the public API.
Test the runtime type to filter on a specific element kind, for example
context.ClickedObject is INode node. Depending on the surface the handler is
registered for, the value can be an INode, an IPort, a
Wire, an IVariable, an IState, an
ITransition or an ICondition. A subgraph state is an
ISubgraphState, which derives from IState.
Elements with no public representation, such as placemats and sticky notes, also report
null, so a null value does not guarantee the click landed on nothing.