REACH-82 "Null value cannot be cast to double"

When Number node is created it has `Value` is `null`. And only rendering causes updating it to `"0"`. Or changing `Value` from "0" to another number. That's why in headless mode when we created node number without changing its value (we need exactly "0") and connect it with another node, running is failed because that other node needs number rather than `null`
Benglin, I'd tried to initialize the value in the way you suggested but it doesn't fix the problem. It needs additional calling 
``var idList = new List<string>();
_parsed = ParseValue(_value, new[] { '\n' }, idList, _convertToken);
State = ElementState.Dead;``
in constructor (it's called in setter). In this case `Value="0"` looks much more better