Using ruler as output node causes crash

[lassoan]

We should not show nodes that cannot be transferred; or at least not crash but log an error if user attempts to send a non-supported MRML node.

See details here: https://discourse.slicer.org/t/openigtif-trajectory-message/8280/2

[Sunderlandkyl]

7f42e19 should fix the crash.

The reason that the vtkMRMLAnnotationRulerNode is shown in the node selector is that it is a subclass of vtkMRMLModelNode, which is one of the supported types.

Do we want to send the ruler as a model through OpenIGTLink?