New exception form can be removed from the DOM by the user

[TheoChevalier]

That's a bug I introduced in #107 but I don't want to forget about it.

\ Steps to reproduce:

• Add an exception to a rule
• Delete this rule
• Try to add a new exception to another rule

\ Actual results: There is no form to add an exception

\ Expected results: There's a form.

\ What's going on: Rule node is being deleted from the DOM while Exception form is still somewhere in this node