Global Variable shows null

Noticed a Global Variable (MinOutsideTemp) not changing in a rule.

Looked at the GV list and saw this

and at the bottom of the GV list it was asking for a value for MinOutsideTemp

I tried entering a value then this popped up


Looked at the logs which revealed...

I can't remove or delete the GV. How do I fix this? I added a new GV to replace the corrupt one and all is working fine it's just that I cannot cleanup the bad one. I do not know what caused the problem. The rule is very basic ie...

Any help would be appreciated