1) Enter edit mode in the UI, then click on the small icon in the lower bar.
2) I guess that's because NULL is not a value and can't be stored in a variable.
When the variables evaluates to NULL in the script, it should be removed completely (but with the exception stated in my previous post, when the variable once has reached the UI). You need to aks someone from Qlik to get answered why it is this way.
In the UI, you can only assign an expression that evaluates to NULL, but not NULL directly.