Confusing handling of None

[ceball]

Here are some examples - there will likely be more problems than just these.

Displays value of 0 when it's actually None:

Also note that because dragging doesn't change the value if dropped at the original location, it can be even more confusing:

We're not looking to support selecting None as a value in the GUI, but probably None values should be removed by interaction.

[jbednar]

Fixing both of these would be great, but the first is probably the priority.

[ceball]

Same for boolean/checkbox:

After select and deselect:

[ceball]

Range:

(although I think if you drag min and max a value gets set)