I just modified an existing rule (present for several days). I added an action for a button being 'held', not just pressed.
After modifying the rule, the normal RM4 page showing the conditions/actions for the rule was shown.
I then deleted the rule, and upon pressing "Done" to delete the rule, I got the above error.
I am 100% certain I did not press the 'back' button or reload the page during the rule edit.
The rule in question can no longer be loaded. Other RM4 rules seem fine.
The system log shows:
java.lang.NullPointerException: Cannot invoke method tokenize() on null object on line 305 (mainPage)
C5 Hub, running 2.1.7.127
Firefox browser under Linux.
Time to reload last night's backup... 
You don't press "Done" to delete the rule, you press Remove.

So, you got this error after hitting Remove?
If you deleted the rule, why do you think you could load it again?
My error in describing the sequence of events.
After modifying the rule and seeing the complete list of actions for the button devices, I deleted the action that was just added, not the entire rule. Upon pressing 'done' to delete the last action, I got the error listed above.
The named rule still appears in RM4, but cannot be loaded.
What action did you remove? I wouldn't load a backup, i would just delete the rule and recreate it. There isn't anything that can be done at this point to recover it. Unless you can reproduce the clicks exactly, there's no way to know what went wrong, unfortunately.