EDIT: not sure why it doesn't display but in fact the value was equal to FFFFFFFFFFFFEFFF which converts to -1.7976931348623157e+308 which I admit is a very silly number. It would still be good if this value showed in the debugger but I don't need it fixed now.
Explanation: In U++, double values < -1.0E+307 are considered Null.
Null itself is defined as -1.0E+308. But to be sure about double equality issues, there is "less" instead of "equal" used. I believe that in practice, limiting double range to e+/-300 is non-issue.