While trying to add support for ARC_WAYPOINTS (36) I found that it was impossible to use the "Unknown" mission item. I get the scary looking error shown below.

It's very easy to re-create this issue:

• start SITL
• on the Plan screen, push the "Add Below" button
• in the Command field select, "Unknown" from the drop-down
• when the "Mavlink ID" box appears enter "36"
• push the "Write" button to save to the vehicle, the error message above will be shown

It seems like the "Unknown" command support must be completely broken